Boobs

Why do my boobs go basicly flat and fall in my armpits when i lay down even if i have a bra on? Even when im standing it seems like there is a decent gap between them. Im not a busty girl im only a 34B. Im self conscious to tale off my push up bras during sex because i dont like how they look when i lay down even tho my man doesnt care.... Does anyone else have this problem?