What Causes Irregular Spotting With Severe Pain?

I had protected sex Aug. 12 but the condom broke and we knew nothing about it. I expected my period Aug 19 but it did not come. I took two HPTs, one was negative it was taken in the middle of the day, other was taken the next morning when I first woke up and it was defective or something because the line didn't show up at all. I started experiencing cramping, which is not normal for me to experience during my period at all, Sunday. Last night I seen spotting. This morning, it's like I have a normal period except the pain is unbearable. Could this be a early miscarriage or am I just paranoid?