I just got a new bike. I absolutely love it but i am experiencing lower back pain. The bike before this was a 15.5 and felt that was too big. I couldn't maneuver it well and I felt it was too high, and became squirlly trying to get through the trails. I thought it was my inexperience but after 3 years and still not working out i deceided to try a new bike. I stumbled upon a trek wsd x-cal that was 14.5 and fell in love! I am just at 5'3 with a 26in inseam. this bike so much easier to ride but after riding about 2 miles on pavement, an hour later i started to get pain in the lower back. Now my old bike which was a trek cali sat upright too much and when i raised the seat the geometry was off and it was too high! this new one i feel like im in a normal mountain bike position. Is this pain from not being used to riding like this or is this bike too small? I don't feel its too small. Maybe i am overthinking.

There are many experienced cyclists here who can offer remedial advice. Many are pros in the trade. If you could post a photo or two of you on the bike that would be the best way to get advice.

Yes, a photo or a few photos of you riding the bike would be most helpful. Get someone to take images of you from the side as you ride by them or use the self-timer on your camera nad a tripod if you have them..

Cheers

Best would be to get a fitting. It is amazing what a few minor tweaks can do to add pain free miles.

I have arthritic spurs in my back.... and lower back pain can be caused from tying my shoes. Your back pain may, or may not be related to cycling, or your bike fit. A small dose of my favorite pain reliever, some ice, and a good night rest will generally fix my pain if I jump on it right away.

Get a bike fit.... professionally through your LBS, or ask a friend to help you through the steps using YouTube videos. Then take your time breaking in to riding again. Have fun!

Lower back pain often indicates that the spine is not straight down there. This is not definitive, but it's worked for me and for others who have tried it. Sit on your bike with a straight back. Don't slouch.

This change in position can help you do that. Move the saddle back on the rails, say a centimeter or two (and lower it a little less if necessary). This gives the lower back a little more room to stretch out and moves your center of gravity away from the handlebar.

Some riders just slouch out of habit and some slouch because it seems to solve a balance problem. Either way, it's bad for your back.