Last period was 8/15. 9/4 i woke up with cramps & then spotting while and after having sex. the blood is heavier now with cramps and lower back pain. ?

See details: I am going to assume that you have regular menses otherwise. Therefore, this bleeding could be associated with multiple factors such as infection or dysfunctional cycle (you didn't ovulate). I would suggest you make an appointment with your gynecologist for a proper evaluation.

Pregnancy tests: Get a pregnancy test now and in a week. If either one is positive you need to be checked out with an bHCG and ultrasound.