Brown discharge.

My app says I missed a birth control pill and I am unsure if I did. I had sex on the 12th, unprotected, he didn’t finish, and I had some brown discharge on the 18th. I can’t remember if I missed the pill before or after the unprotected sex. So I took 4 pregnancy test and they were all negative. Which is what I want. I don’t ever have a period, so if I “miss” it I won’t know. Any advice? Could the brown discharge be from me missing the pill? I have no other symptoms of pregnancy. I’ve been stressing and I need my head to be cleared.