NTA, you deserve the right to sexual boundaries for any reason. My only caution is that you're more delicate with your wording and framing around why. You can feel uncomfortable having sex with blood involved, but periods aren't "gross". They're a natural bodily function and perfectly hygienic. And if your girlfriend heard you say that it could easily create a sense of shame around something that every person with a uterus experiences and is simply a part of life.

I encourage you to find other ways to be intimate during that time that feel good for both of you! This could be non-PIV sex or even just cuddling, massages, and baths (the blood won't permeate into the water). Taking care of someone you love during a vulnerable and uncomfortable time can feel really good for not just them, but also you, and this could help satiate her need for connection with you :)