Yes get it checked. I did have truncal luymphodema post radiation and definiately my boob was swollen. It didn't appear larger however it should have been smaller as I had a lumpectomy. Now that the swelling has gone it is slightly smaller and I am no longer symetrical!!!
It is normal to get swelling post radiation however get it checked as they can give you advice on how to reduce any swelling and of course put your mind at ease.
During my radiation my fluid levels did build up and was staring to back up into my arm. (not to the point of being diagnosed with lymphodema in my arm but was close) Manual lymph drainage definitely helped and after I had finished my rads a few months later all my fluid levels were back to normal in my arm although still some swelling on my boob. Took some months for that to get back to normal and I still get some swelling and bra marks but nothing major.