why this question in infertility section :) anyway; do you work on a computer everyday?
tight muscle could be the issue. and how bad is the pain?
it could be rotator cough damage which is quite common and i had the same issue. i got confused like could that be cardiac but my EKG was normal :) see the doc and get tested just to be doubt free.
Probably means nothing. Maybe some muscle strain, or indigestion, or both. Talk to your doctor at your next check-up. In the meantime, try Mylanta and see if the chest stops hurting.