I'm 25 and experience it quite often. In my case, I find it harmless...other than a shooting stabbing pain when breathing. Just take soft easy breathes for a short period. I did notice that I got it more and more as I progressed in my pregnancy. In any case just to be certain, always ask your doctor! Hope you feel better soon!
Take a couple fingers and gently probe along your ribs on that side and see if you don't pinpoint a small area that hurts really bad when you push a little on it. If you find such a place (or a trained medical professional does), consider whether or not you may have costochondritis.... do a search on it and see if you think that sounds like your situation. Make sure it isn't anything more serious with a health care professional. If they diagnose you with costochondritis, they may prescribe a few days of cyclic anti-inflammatory medication and suggest rest of the area and moist heat on it.