I need to swim but I'm on my monthly!?

Hey Girls! As you can read I am on my monthly, (my period)! This Friday I have to go swimming, well not really have too though. My Best friend invited me to her pool party (girls only) and i got my period! I could use a tampon but it hurts me too much. I get all numb and dizzy, after wards I get a really bad headache. I don't want to be a bummer and hang around the pool side. Please Help! Thank You!