Hi girls, I need to someone read my feelings and worries, hope you can help me.

I've been anxious since the last time I had sex (June 11th), I'm on the pill, I take it perfectly, never missed one. I had my withdrawal bleeding on the date it had to come and also I took a test 3 days before my period and it came negative. But I still feel anxious and I think I could be pregnant, why? Why does this happen? Can someone relate?