I'm 21 and weighed 210 (+/-5) at the beginning of the year. Got the gym membership in March. Started running and watching food in the summer. Hated running, and got the bike last week of August. Also am type II Diabetic, but my numbers are awesome and I can eat more now since I try to put in an hour of riding 5 days a week. I weight 178 now. As to where I am in my training, I've only put in like 300+ miles so far, so not much. Did a ride with a dude (how cute-- my first bike friend) the other day: Around 19+ miles in an hour with an average speed of 19 mph.
Woke up early at 7 and just kind of got up and went. I had eaten at 5pm the night before and had an apple at 11pm. Didn't drink much that night either. Had a little bit of Powerade (which I never usually) before I left. I normally eat/drink very well before my regular rides and I was so sleepy that I forgot to kind of bulk up that morning. I think this was Bonehead Mistake #1.
Got there and did 15 minutes of spinning. Kind of cool-- figured out spinning form and all that stuff. I'm going to put my normal cadence at least 80rpms because I'm a poor music student who uses his metronome as a guide for cadence. I had to be doing 105-115 at this class. After the 15 minutes, I was sweating up a storm and started feeling dizzy. I said "F this" since I'm diabetic and didn't want to kill myself, so I tactfully salvaged my pride and left (of course asking when the next class was).
My friend: My poor squeamish friend. "Megan, keep walking", I said, calm as ever. "What?". "Megan, keep going," as I then proceed to throw up blue in the bushes. Never threw up just plain old blue before, but it was kind of pretty.
Blood sugars were normal, so I wasn't like going into shock or anything.
So I'm thinking this was just a baby bonk or something which could be chalked up to:
1) Not eating enough
2) Not drinking enough
3) Spinning faster than I'm used to
4) Being used to spinning