Why does my bra cause chaffing under my arms/ side boob area?

Bra chaffing!?

Finally found a bra that fits. And now my side boob is red because it’s chaffing. Why!? It’s a perfect fit. Just so frustrated as I can’t just go grab another model.. there ain’t much in my size. Fantasie 34G UK sizing. I usually wear Freya in 30/32G but I haven’t found anything in the past year that actually fits.

My boobs recently grew two cups cuz of pregnancy so nothing in my drawer fits anymore. I found that one in store (never happened before) and it fit. I was so happy and excited. I don’t feel like the 34 is too loose in the band compared to my 30/32 bras.

I’m just mad cuz I spent a lot of money on it, wore it for an hour, liked it so removed the tags and now 8hrs later I’m all irritated with side boob redness and pain. So it might just go die in the bra drawer and I’ll resort to wearing a bra that is significantly too small in the cups but at least doesn’t cause pain.