is it not so bad to play with damaged tendons ? sometimes I think I stretch my fingers too far, and I cannot play the next day. Then I just play pieces or exercices that don't need much stretching.
Just to re-iterate Ade1967's post: Don't play if you are in pain!
Stretching is supposed to stretch muscles, not tendons. Tendons cannot be stretched. If you do, you will damage them. Therefore, stretching has to be done in a very controlled way, not in a haphazardous way. If you believe you have damaged your tendons because of stretching, you are in serious trouble. I assume it's something else. Why can't you play the next day? Is it because of muscle fatigue or because the area where the tendons are hurts? There is a big difference between stretching the tendons and causing inflammation because the tendons rub against something.
Everything you are saying points to a flawed technique. If you have problems for 2-3 weeks after practicing octave termolos, you are practicing way to long and in an entirely wrong way. Nothing you do at the piano should give you problems for 2-3 weeks!
Problems with octave tremolos almost always come from contracting muscles that don't need to be contracted, such as holding the middle fingers stiff and cramped (assuming you are playing the octaves with fingers 1 and 5), whereas they need to be relaxed, as well as fixing forearm, wrist and hand in a cramped postion whereas they need to relax a bit when you move from one position to the next.
I would strongly recommend to evaluate, or have evaluated by an expert, very carefully what you are doing!