Pumping

Someone please help me, I don’t know what size flange I should be using. I was using a 28 but then my boobs/nipples were really hurting so then I thought it was to big then I switched to a 21 and it seems it’s doing the same thing, I just feel like pitching when it’s pulling. I also feel like both sizes pull more than just my nipple. I have really small nipples, like they hardly stick out (almost flat) so feel like that’s why both sizes are pulling more than my nipple. Is there such thing as having to small of nipples to pump? Can you pump with flatter nipples? I’m just so confused on how these flanges should fit.