I've had my MTB bike for a few years now and have never done big miles with it. Just use it to ride the trails with the kids for under 20 miles. I have had this problem ever since I got the bike. DH bought it as a surprise for me, so I didn't get to pick it or get fit or anything. Problem is when I ride the back upper part of my shoulder hurts. Like just inside of the shoulder bone. It feels like I am stretched out too much, but I'll try to straighten out and move around on the bike and nothing helps. After a few miles I get used to it but then it comes back towards the end. DH says it's because I don't ride my bike enough and haven't gotten used to the cycling position. I feel I have rode enough to know it's not the right fit. I can't afford to pay the $100 our LBS charges to get a fit. Plus in another year I'm going to be getting road bike (with a true fit). But I'd still like to have my MTB to ride around the trails. I'm also getting pregnant next week (gestational surrogate) so won't be able to take the bike outside, but can try it on the trainer.

Any suggestions to any easy changes I can try to make it feel better? Is there a rule as to far the arm should be straight or bent?

Thanks! (and if there is a better forum, feel free to move this, I couldn't decide)