Hard to know without seeing it.
I started having trouble with my binder after I started T. It was before the weight gain so it wasn't tightness. I had a rash all over my chest that only hurt and itched (mostly hurt) when I exercised or got sweaty, which was often. I tried hydrocortisone, coal tar, salicylic acid body wash, not sure what else. Finally I started only wearing the binder with a shirt under it. That's not the most comfortable solution in the world, especially when it's not cold outside, but when I started doing that my rash gradually cleared up.
None of it happened before T, so I wondered if it changed the ph of my sweat or something. I'm not sure how to deal with summer. I guess I'll wear a wife beater under the binder. Or something. Luck to you.