What Causes Delayed Periods Along With Backache And Sore Breasts?

Hello , I was 20 days late with the last 7 days having sore back cramping sore boobs fatigue nauseous and irritable , done home tests and blood test all negative , I yesterday got my period very light pink and sometimes brown spotting , then was the same over night and today I have had severe cramping not just period cramps but feels like my whole stomach is tensing very tight feeling above my belly button and be between the opening of my ribs, I am now bleeding heavy with clotting a dark red colour. All my blood work was normal no std of sti no urine infection haven t had a history of cysts or pcos never have had an irregular period it is every 28-30 days every month. I am sexually active with my partner & no protection is used. I m a 22 year old with a healthy lifestyle Could I be pregnant at all or because I have heavy bleeding now it means I m not ?