Welcome to the forum and thanks for sharing your story. It's great to hear from survivors who've been NED for as long as you have! I hope yours is just a scare and that everything will turn out fine. I've been done with treatment since July 2016, and finished radiotherapy last March (breast and node areas).
I too have had rib pain, on and off, just below my affected breast. It would be a mix of stinging and more diffuse pain in that whole area. The pain would actually not be precisely localized, so I had trouble saying whether it came from the ribs (bones) themselves or from the skin/fascia on top of them, but noticed that I felt more pain when I stopped doing my exercises to stretch my whole breast/armpit/side area. Once I started exercising again, the pain disappeared. I haven't felt it for a while (although now it's more my shoulder/arm that feels weird). The way I stretch is that I lay down at the edge of my bed, in a T-shape position, with my affected arm hanging down. I sometimes hold a light water bottle to add some weight. And I stay like that for 1 minute or more. I then stay in the position but bring my arm closer to my head so that it stretches my armpit more.
I am telling you this because I know that radiotherapy can have long lasting effects on our skin and nerves, and that these can come back even after years and years without any pain. You might want to try stretching and see how it feels, and also mention it to your doctor who could recommended a specialized physio?
Please keep us updated. There are a lot of women here that can help you out or simply lend an ear to listen!