My breast was also red and enlarged, but not with just fluid I guess. The doctor said the way to tell if there was a fluid accumulation was to use a syringe to draw off fluid - but there was none there. This was 3 months post lumpectomy and about 6 weeks after radiation (no redness at the time of radiation). It was red in the inner lower area, my lumpectomy was on the opposite side on the lower outer area. I did not have whole breast radiation.
I wore the surgical bra 24/7 for 3 months, then wore a soft but supportive sports bra all the time.

I sent a photo of my breast to the cancer center through MyChart. I think everyone everywhere has probably seen my breast, but I’m 70, not like anyone will want that photo! I live over 3 hours from the center - I’ve sent multiple photos over the first 18 months. They seemed to really appreciate the ability to see what I was trying to explain with words.

They said to make sure there was mild compression on that area. But I finally figured out the redness was worse with compression, and would go away when I went without the bra for a while. My regular local doctor looked at it and said there was no infection, and didn’t think there was fluid build up.
My theory - my vascular system was disrupted with the surgery (I also had a mastoplexy at the same time - so lots of rearranging of the tissue) and the compression brought blood to the surface just like it does on any other part of the body (pre-bruise). So I stopped adding compression. And it went away in about a month.