Help - Rebel 300 destroys my back
This is an odd one. I've owned any number of mountain bikes, waverunners, etc. I picked up an old 90s Rebel 250 to wrench on and learn to ride, and after a few weeks of riding decided to move up.
So then I bought a pristine Rebel 300 to ride on.. and if I ride more than 10-15 mins, my lower back absolutely kills me the next day. I'm in pain for 2-3 days after every ride.
I can't explain this. When I am on the bike, nothing feels particularly wrong with my riding position to me. I'm not uncomfortable, and while the rebel 250 felt small (I'm 5'11) the 300 technically feels like I fit better - only complaint would be that my legs feel a bit cramped/pushed up.
Does this sound familiar to anyone? Any ideas? I hate to say it but short of some brilliant suggestions I'll probably have to sell the 300.