Hello. I can definitely understand your concern. First of all, I would phone your doctor as soon as possible.
Secondly the brown blood, they say, is old blood. Blood can stay in your uterus for a number of days before making it's way out. It could have been implantation bleeding.
What does concern me is the cramps. How bad are they? At the same time, just read that you've had them on and off throughout your pregnancy. Is this your first? Could be the uterus growing
Make sure you do call your doctor and do take it easy today.
Thinking of you.