ESH and with that i address almost everyone commenting here.

What the fuck is wrong with you people?

To OP:

Communication is key. You're 16 so you got some time to learn that. But nothing beats to the point, honest communication between partners. If you're having any sort of issue with your body that's comparable by impact on daily stuff to a period, you'd want your girl to know that, right?

It's possible it will still gross you out or you'll be uncomfortable, but that doesnt matter. You know what would make you even more uncomfortable than talking about periods? Having to help your gf out when an "accident" happens and she never mentioned that she's on her period because you told her not to.

Comfort her and avoid more uncomfortable situations by talking to her about it. It may not be your responsibility but it will certainly still affect you even if you close your eyes to it, so make the most out of it.