So, I'm 19 weeks pregnant and went to get bra fitted recently at a department store because my boobs were really uncomfortable and falling out of my old bras, which were a mixture of 30Ds and 30DDs. I was fitted as a 30D/DD regularly for the last few years.
The bra fitter said I measured a 28 as the back size, but that I would really struggle to find bras in that size so she'd stick with the 30 that I was already wearing and tried me in a 30E. That fitted well so she said I was technically a 28F but a 30E was still ok and it would be easier to find bras in that size. She was right as I couldn't find any 28Fs in the store but managed to find a few 30Es.
I've since bought two 28Fs online from a lingerie shop, and am very confused. One of them just didn't fit at all - one of my boobs was falling out of the cup, which I was very confused about given the cup size is actually bigger? 
So that one is going back for sure.
The second one seems to fit fine on me but the band feels very tight and like it's digging in a bit. I've never worn a 28 before (well, not since I first started wearing bras in like AA cups when I was a child!), should I just suck it up and will I get used to the feeling like i'm wearing a corset? Is that how a well-fitting back size of a bra is supposed to feel? I feel much comfier in my 30s but if I measure a 28 back surely I should be wearing a 28?? I don't know whether it's just because I'm not used to it and this is how it's supposed to feel, or if it's not supposed to be this uncomfortable and she was wrong and I'm bigger than 28?