Can't tell if my bra fits

Is it normal that I can't tell if bras fit me or not..? My mom has called me stupid for not knowing if they do, but I really just can't. Sometimes I'll buy a bra and think it fits perfectly and after a few hours of wearing it, my entire nipple will be popping out. And I still can't tell if it's too small or too big. Sometimes I'll have a bra, and there will be a gap of space between my bra and my boob because the bra is too big I'm assuming, but then my nipple will come out like it's too small. Maybe I just have really big nipples/areolas and that's why they always come out, but I still can't tell if the bra is fitting and why or why not