The girl I'm dating has a trend that is backed onto her cycles, we've been together for 4 months, and everytime her cycle starts, she changes dramatically. I'll usually see the change and ask why she is acting (upset/distant/grumpy) and she'll always hint that this is the "time of the month". We go from seeing each other 3-5 times a week during the other 3 weeks down to like 1 during the time of her period, and usually its a tensioned hangout at that. And when I ask her to hang out more or get frustrated when she doesn't want to get together, she gets upset that I want too much of her, questions the relatioship, etc. This is the first time I've finally connected the dots that her mood/attitude is related to this every month or so. During this time she will usually opt to see her friends more (which is totally fine), or simply sit at home and do nothing as opposed to want to spend time with me.
I ask because we just had a long weekend, and I saw her once, for a few hours, then she asked me to leave so she could have a nap. It was a nice long weekend weatherwise and we had some small plans which she didn't so much as cancel, as just avoided altogether.
Once the cycle ends, things return to normal and we go back to being together a regular amount, spending quality time together, having fun, etc.
I don't understand why she is so extreme, I've never dated anyone like it.
EDIT
Furthermore, as a guy, what can I do to help her, or myself, get through it so that I don't let it bring me down when she is like this? Its hard to work with her when she simply doesn't want to hang out.