[Girls...help?] How am I supposed to enjoy fourth of july today when...?

Today is my heavy period and I can't go swimming with my friends and I feel moody and uncomfortable? The party starts in about an hour or so, and I still want to go...but I'm afraid that I'm not going to have any fun. Does anyone have any advice? Sorry if I sound so childish...I'm just feeling so yucky right now. :-P