Help With Symptoms

I'm 15, I've had my period for 3 years. It's always been regular, 6-7 days and it starts when planned (may be off a day or so but it is typically normal). Recently, I've felt really sick a few days before I start my period. It's messing with my swimming, making it so I cannot swim as well as the rest of the group and it puts me so far behind. It's messing with my digestion, giving me bad cramps, and making me feel extremely agitated. My periods have always been heavy, but now they're heavier & I experience more clotting. As someone with no idea what will help, can someone tell me what (if I can) do to make me feel better?