Skin changes post radiation

After radiation on my left breast I noticed a large 15cm bruise like mark on my back (same side as the radiation treatment area).  My radiation oncologist didn't believe it had anything to do with my radiation and she suggested I use a topical steroid cream to see if it clears it...it didn't 😒 I tried an antifungal cream as it is a little itchy however this also doesn't seem to be working.  I was wondering if anyone had experienced such an issue?  I understand my immune system has taken a hit and quite possibly it's just an immune response from treatment. I'm just perplexed as my breast skin didn't change at all post treatment.  Thanks in advance 😊