2 and half year old refuses to let me clean her down bellows

I have a 2 and half year old step daughter. She is still in nappies because her mother doesn't want to potty train her, that's her choice and I will continue to keep her in nappies until her mother chooses otherwise. The issue I am having is cleaning her down bellows. Recently in the last few months she closes her legs shut firmly, and refuses to let me clean her vagina. It's not really been a massive concern of mine because we bath her regularly etc and i didn't think it could get dirty as such inside if she's bathing. (When in the bath however she still refuses to let me clean her vagina only her bum etc) I ask her could she do it herself and she obviously can't do it properly but it's the best I can do given the situation. Recently she had a really bad poo and it was inside her vagina, I begged her to open her legs and explained that there was poo in there, she wouldn't let me clean it until I agreed I wouldn't open her vagina just clean it. (Had to give her a bath to get it all out) she keeps saying it hurts and literally will not let anyone clean her there. long story short... should I go to the doctor about this? Is this normal behaviour for her age? She literally is nearly in tears when I wipe the outside of her vagina with a wipe. Open to any tips on how to make this easier etc? Me and my partner didn't think much about it before.. but now we're starting to get worried :/