My best guess as to why she keeps talking about a topic that obviously makes you uncomfortable is to get a reaction out of you (AKA trolling). Stop reacting, she probably get bored and stop teasing you so much.
However, you have to understand that even if decides she will no longer torment you with this subject, that doesn't mean she's going to stop talking about her period entirely. She isn't unique in this, most women I know, myself included, will talk about it from time to time, especially if we are having a particularly painful period.
Let me put in in perspective for you. You know the painful cramps (charley horse) that come as a result of getting dehydrated after playing sports? Take that pain, multiply it by 10, and endure it for about a week while have to go about your daily routine. Yeah. You'd complain too if it were you.
If you intend to continue dating this girl (or any other girl for that matter), I suggest you suck it up and get used to it. If nothing else, be thankful you only have to hear about it, not actually live it yourself.
Besides, I'm sure you've had conversations with her that she didn't find particularly tasteful. Don't lie.
