She's offering to do things for me that she HATES?

I HATE the idea of anyone doing anything for me that they do not want to do - even the smallest favour.

I absolutely HATE the idea of anal sex (sorry for the details). The idea disgusts me 100% and I do not know why so many men love it so much (I'm male btw). My gf told me that she's tried it once and didn't like it at all. So far so good, right? No problem.

BUT then a few weeks ago she was REALLY upset by something that happened at uni and she told me she needed to learn to say "no" (apparently, she agreed to send another student all of the notes from a class he'd deliberately not come to). I told her to practise on me, but she said she wouldn't feel right. So I decided to throw something her way and see how she reacted. She KNOWS that even though I've never done it, the idea of anal sex disgusts me AND I know that she didn't enjoy it. But I told her I wanted to try it "just to see" and she replied "if you really want to, I'll try it with you".

This really scared me and I told her that it was just a test. WHY is she willing to do that? She told me she DIDN'T enjoy it. Even if I DID really want to try it, I would NEVER suggest it, knowing that. Sex is a MUCH more invasive process for a girl and the fact that she's offering to do something she hates (not even "didn't enjoy that much", she actually used the word "hate") just for me worries me. How many other things is she doing for me then that she doesn't enjoy? I'm now finding myself second-guessing EVERYTHING with her in case it's another thing where she's actually not enjoying it at all, but just doing it for me.

I would MUCH rather go without, myself, than see her put through anything she doesn't enjoy. I'm freaking out here, because I love her and HATE the idea of her doing ANYTHING she doesn't want to do, or throwing out her own schedule for me. What can I do to calm down and ignore this VERY BIG worry of mine? Thanks in advance.