Help me!

Okay so this ONLY happens when I'm on my period. I don't know why it happens but it does and I dread it every time. Okay so, my diet stays the same when I'm in my period, but I always seek to get diarrhea! And the worst part is, is that sometimes it's not diarreah and everytine I go to go #2, it's an extreme pain in my anus. Like a knife coming out of it! I can't handle it sometimes, to the point of screaming in agonizing pain. Should I go see a doctor? Even though it's only once a month, or what? You ladies have any idea what or why this is happening?! Thanks and so sorry for the gross post- but I need help and relief! Xoxo