1. This is a good thing. It means your girlfriend isn’t pregnant or so overworked and/or undernourished that she can’t go through a very normal bodily process. These scenarios probably require a little follow-up on your part, and both are thankfully above my pay grade.
2. This isn’t really going to impact you for a significant percentage of your intimate life. Usually, there’s going to be five to nine days per 28 day cycle where your girlfriend will be directly impacted by cramps, her flow, and bodily discomfort.
3. This isn’t a big deal. As in, if you don’t live together or haven’t been dating for longer than a couple months, don’t panic if you have or haven’t been updated on the progress of her cycle. If she seems otherwise excited to get to know you and you’ve started to have sex or are planning to start imminently and says that she can’t hang out on a particular day, don’t push it. Just offer to go out later.
4. Leave tampons in your bathroom if she stays at your place often enough to leave a toothbrush. This is also a generally thoughtful thing to do for that 35% of the population that might come visit your apartment. If she’s already brought some over, keep a mental note on the brand and type, or just ask what she prefers.
5. If your girlfriend says she’s sore, hurts, or just asks for Advil, offer her Advil. You would do this if she had a headache, wouldn’t you?
6. Period sex. Seriously, if she’s complaining that cramps are killing her, offer to have sex and suggest it might help her cramps go away. Cramps result from pelvic muscles pushing ova and uterine linings out; sex and the correlated thrusting, penetration, and orgasm(?) stretch those muscles out the other way and clear out debris. It’s like a warm-down. If height allows, have sex in the shower. Otherwise, put a towel down, have her on top, and shower after. If this squicks you out, think of it as extra lubricant.
7. Chocolate chocolate chip cookies. Women on their periods craving chocolate aren’t hysterical; they’re trying to replace a lot of nutrients and energy regularly flushed out. Framed positively, learning how to bake things with chocolate and doing so on a regular, semi-frequent occasion is a really easy way to do something nice for your girlfriend and get credit and praise very disproportionate from the effort you put in.