2nd day of period have unbearable cramps, very thick, stringy, very dark clotty almost black discharge never had this before I am sexually active?

Heavy period. There are many options to treat heavy menstrual cycles. If this is a one time thing then the cycles usually return to normal. If you continue having heavy periods consider one of the many birth control options which will regulate your cycle and in some cases minimize bleeding and cramps. Ibuprofen is very effective at treating cramps.