Every time I am on my period I get sexual feelings for my best friend who is a girl. (I am a girl who is attracted to men) But it is only during that time of the month. It's gives me anxiety and I have high Axiety already.

Why does it bother you? Many women are bi-sexual and others still wonder about it.
If you want to act on it? If you don't want to act on it, just enjoy the feelings, when you have them, and let them pass later. Don't worry.

I think your key here is don't worry. Why should you when you know the feelings will change soon? I wonder what you even are worried about? Maybe you should just realize your anxieties reach a peak during certain times of the month and level out after. Most of us have little control over what feelings we have or when, but we do have control over what we do about those feelings.

Nothing wrong at all with a "straight" person finding same sex people attractive, it's normal, I think, especially a same sex friend you love, and admire as a friend.