Help please???? what do I do?

I just started my periods a while ago and have stayed away from the pool and other stuff, but I'm on my period and my mom is forcing me to go to the beach tommorow, not caring I'm on my period, she doesnt want me to wear tampons and she is pretty much blaming me for not going to the pool since thursday, when I started. She called me a liar when I said my cycles are 7 days, because hers are 3. What do I do? I have to go to the pool tonight, and beach tommorow with my friends, they wont understand why im not in the water, we arnt close enough to talk about these things. Im scared they'll pressure me into the pool then I'll have blood running down my leg. Any help is appreciated