My husband is prone to tendinitis - all just below the elbow. It never bothers him on the roadbike, but almost always is irritated by the mountain bike.

Are you riding off road, bumpy roads? You have to use your upper body on a mountain bike more if you are offroad and this can cause some tendinitis.

It is hard to tell what is going on when you are talking on the web, go to the doctor have it checked out - then go to a fit expert to try to minimize the problem.