It's on the outside of my right knee but aligned with center of the knee cap. Pain was most present when I cycled but not so much outside that. I don't feel it when walking but sometimes I did when bending my knees out of a chair. I haven't cycled a week, and I don't feel it now. What was it, and what should I do? I should mention I first felt it doing an indoor cycling class. I don't know what triggered it. I'm thinking the one legged pedal stroke with too much torque, but I'm not sure.

Sounds like the saddle is to low I would suggest moving it up and getting a proper fit. Sheldon brown has a guide on saddle adjustment were he suggest slowly raising the seat to the point of being to hight. Then lowering it a bit although for anyone that rides a lot or competively I would suggest a fit.

I just realizes you said indoor cycling class adjust the seat

I don't think saddle height has anything to do with it. I've been cycling two years. My seat is closer to being too high than anywhere close to being too low. If anything, the seat might have been a bit far forward when I started the indoor machines, but I've adjusted that. Any idea what I could've injured?

As far as injured I am not a medical professional so I couldn't speak to that. Your situation is similar to what I was experiencing on my road bicycle. What I had to do was adjust the clips and saddle height, I am not sure if you are using those in your class or not. If you are I would play with the clips or have your instructor help you with the clips. When I was fitted for my road bike the fitters noticed that my knees were bending awkwardly as I was riding and I had the saddle and clips adjusted. I also experienced the same pain as you did, did not hurt while walking or sitting just on the bike. I also experienced the pain only in low gear high torque situations. I would suggest seeking medical attention if it is recurring pain or severe enough to stop a simple workout. To fix the problem, if your in an instructor led class with clips. I would have the instructor look at how the bike is set up clips etc. If not in a class I would work with a trusted work out partner or consult the gym and see if they have some one that can help with the setup. Cycling should never hurt your knees, it can actually help with any knee problems.

Yes, we have clips. I was taking those classes for a couple months and never had any problem except toward the end before the holidays. I'm thinking I either did something stupid, or it was from overuse. I tried to make the fit as close as possible to my outdoor bike which I never had problems with.

I would look at the clips, something simple as switching from a peddle with a lot of float to a zero float peddle could have made the dramatic difference between no-pain and pain, A change in cleat placement could have caused it.

ITBS is what you have. google it or search this forum

I did 10-11mi today, because it's been so long. The pain came back. Getting out of a chair does hurt a bit. Getting in one doesn't. I'll try some stretches.

Most probably so. This is not easy to get over. "Some stretches" won't do it. The following links contain an ITBS test and treatment.
https://en.wikipedia.org/wiki/Iliotibial_band_syndrome
https://www.itbs.info/html/treatments.html
https://www.sportsinjurybulletin.com/...e-injuries.htm

It also would be a good idea to lower your saddle a little, say 1/2 cm.

I've been trying this stretch.
https://www.bikeforums.net/www.youtube.com/watch?v=M0e8FPL787E When I ice the outside of my knee, the top/outside part of my foot before the ankle goes numb. I'm not sure sure if that's bad. I feel it around 10min and I ice for 15min. I don't know if it'll help, but I'll lower the seat a bit. I have flat arches, and I overpronate when I run. I'm not sure about leg length discrepancy, but my right foot is for sure bigger. When I coast I usually fully extend my right leg. I'm not sure if that has anything to do with it.

in addition to what has already been mentioned.....
-cleat placement (incorrect inversion/eversion of foot, or cleat placed too far medially under the foot). try using the flat of the pedal w/o using the cleat or clip and see if it makes a diffference.
-Foam roll to break up Myofacial adhesions in legs and hips ( don't just pay attention to the IT band- the problem could be in the hip or the muscles in either sets of muscles above and/or below the knee- e.g., quad/ham to gastroc- that can pull the knee out of alignment under stress. Issues w/ the hips frequently show up as knee pain.
-saddle placement - not just height. too far forward can also stress the knee. ditto for pedaling out of the saddle- which you are likely doing a lot of in a spin class. long periods of pedaling out of the saddle can produce pain in the front and/or sides of the knee because it produces stresses that are unlike those those which occur during walking/running. learn proper pedaling technique. the problem will be less noticeable as the stabilizing muscles around your knee get stronger.
-bad technique (guessing most likely) - do your knees move inward or outward as you pedal? spin class was most helpful because I use the bike in the front of the room and watch myself in the mirror to look for bad habits. put a bright sticker on your knee and watch how you're pedalling- does the sticker move in a direct line up and down or does it move in either direction sideways? Be aware of what muscles you're using. If your quads are tired/burning at the end of a session then you're over using them to produce the pedal stroke. this can cause anterior knee pain. learn to balance your pedal stroke.
-Your description is consistent w/ over use issues of somebody who is just starting on a bike/hasn't been at it that long/doesn't do a lot of it (2 yrs of casual riding isn't very long). dial it back. don't mash the pedals and pull back the torque utill you can do a session w/o pain.
-biomechanical issues related to leg length can be addressed w/ cleat and saddle placement. Start w/ what's obvious. work on your techique.
***check w/ your doctor if the problem persists***

good luck