Period??? Swimming?? What do i do?

I've started my period before, it was on halloween and i've had it about 3 times since. My period cycle is wierd i have one like every two months! I'm going swimming with my boyfriend on friday and i'm scared i'm going to start my period while swimming and make a huge mess! Ahhhh! I'm not scared to wear tampons but i have heard it's not healthy to wear a tampon when your not on your period yet! Does blood come out when you're swimming? What do i do???