Give your head a shake man!!! Either you wanted her to enjoy it or you wanted to hurt her. Which one is it??? I think you have an ego problem and self estime issues where you think that you arnt big enough.... which is most likely the case. At any point did she ask you "is it in yet?"
"I was really disappointed because I thought she wasn't going to be able to take it."
"I might bring this up with her and potentially break up with her if she lied about it being her first time."
These two statements make you a dickhead.
Your upset because she didn't tell you your cock was so big it tore her and since your so insecure about yourself you would break up with her in the possibility she MIGHT have lied.
Let me rephrase that Your an insecure dickhead who does not deserve a woman who will trust you enough to do anal.
Then you shouldn't be with her. You have nothing to gain from stressing about her past. It is HER past... not yours and it has nothing to do with you. Just be with her and enjoy every moment you have with her. If you can not get passed her "potential" ex's then you need to leave her. But you have to leave her because your all messed up from what you have generated in your head and not because she didn't asked you to stop. The problem lies with you 100% and NOT with her.
All of the above....