What Causes Severe Abdominal Cramps After The First Intercourse?

I had sex for the first time about 4 days ago, ever since I ve been getting cramps in my lower belly like sorta close to the vaginal area, we did not use protection but I am on the pill, I ve also been getting back cramps too but not often as my belly is, when can I tell im pregnant? Or are these signs of it, it takes 3 days to start getting symptoms as I was told. Help??