What Causes Irregular Periods And Abdominal Pain?

well.. i took the morning after pill just after i had my period i experienced some heavy bleeding for 5 days which was fine.

Now im late for my next period by 4 days, ive taken 2 pregnancy tests and both are negative. Ive only now just started to experience darkish brown discharge which is light. Its still going on and i have seen some red blood in it. Ive had the usual lower abdominal pain and back pain that i normally get. Is this my period or something else?

Please help. im worried.