No idea!
I have the exact same issue! Often times it sneaks up on me and I'm walking around all anxious. Typically it will hit me that it's close to time for me to start and that will help me relax a little bit. I just tell myself that my hormones are a little off and it's making me anxious. That alone often helps because I can at least attribute the anxiety to somethng concrete.
My doctor gives me a small dose of xanax from time to time. I rarely use it, but I find that *if* I use it, it's typically during this time.
I have anixiety too right before my period. It is due in 4 days and I always crave chocolate, feel weird emotionally and physically and have mild panic symptoms. I hate it. I have to tell myself to relax and not panic. I think this is common.