I had sex four weeks ago, just 4 days before

Patient: I had sex four weeks ago, just 4 days before my period was due to start. I am on Tri-cyclen Lo, and used a condom (it kept slipping off, however he did not ejaculate and we stopped after 15 minutes of this) I had a normal period, however it was 1 day late. This month, I am having a lighter period than normal, but it is on time. I took a pregnancy test 30 days post sex and it was negative. Could I be pregnant?

Symptoms: Headache, irritability

Doctor: