Why haven’t I had a period seven weeks after my abortion?

Patient: I had a surgical abortion 7weeks ago i was 5 weeks and 4days pregnant after the procedure I had bled for a week 4 days after it wasn’t like any normal period it was real dark and light. since the abortion I have had unprotected sex I’ve been having cramps for 3 days but still no period. I’ve taken 4 pregnancy tests since the procedure and they all came back negative.