Ick, mine do that too (i'm assuming you're pregnant? I'm 31 weeks) Mostly when i drive. There really isn't much i can do to make it feel better besides getting up and walking around. I cant sit at my computer desk very long before my ribs start feeling like they are going to break right out of me!
I am 31 weeks pregnant and have this problem sometimes.. But what i found that works most of the time is getting up and stretching out. Just do some light stretches with your arms and take some deep breaths.. It works most of the time for me.. I hope and pray that it helps!!