My period was three days late I had sex and the next day I started spotting but my flow is very light I'm not filling the pad and there are clots this has never happened before is there a chance I could be pregnant

Were you on the Pill? This could cause.

No I was not on any kind of because

Pregnancy is going to be possible if you had sex... especially unprotected sex. It could be your period was just off (it happens) and having sex at the same time caused it to be even more so.
Give it a couple of weeks and try a pregnancy test. Or wait until your next period is due and see what happens at that time.