I'm going to be on my period at my friends sleepover tonight!!! Help!?

I am 15 and my friend is having her sleepover birthday party tonight. The problem is.. I just got my period last night before I went to bed. Today it is really heavy and I feel bloated and crampy. I feel like blah and I just want to relax and watch tv. At the party we are going to swim then come back and hang out the rest of the night. I am really nervous because all the girls there except for 2 I don't know them and they are mean and I'm going to be on my period. I don't want to tell anyone either. Everyone will already be in their bikinis when they get there and I am going to have to go to the bathroom, change, clean myself up and put a tampon in and it will take forever then I will have to take it out and clean it up and put a pad on and people will wonder why I'm taking so long. My period is really heavy right now and whenever I walk I can feel blood gushing out. I'm soooo scared! What should I do?