I had radiation in 1998 and again on the other side in 2003. Both times I had the rib pain that you describe. My mom had radiation in 2002 and also had the same type of pain. The radiologist told me to expect it and that it is normal. Just to be sure, though, I would ask your doctor about it.
Dear Sue: It is possible that you have a "pulled" muscle or that there is some damage from the radiation. However, if you are having pain, the best thing to do is have it evaluated by your physician.
That's among the possibilities. It's really hard to evaluated a specific symptom on the internet. The best advice is to let your doctor and/or your surgeon about it and have an actual exam, possibly including rib xrays.