I Am 16 And Am Having Very Painful Nonstop Cramping.

I am 16 and am having very painful nonstop cramping. Period is just on time, but the last few days before I was having very watery white discharge and quite a lot of it. Started period this morning and it is usually very heavy the first 2 or 3 days with moderate cramping, but today it was a light flow of reddish pink watery blood and i have severe nonstop cramping and am very confused and curious.