Unprotected sex on September 3rd

My boyfriend and I had unprotected sex (I am on birth control though) on September 3rd. A couple of days later I had blood in what I think was in my urine. i went to the doctor and she did a urine test and it came back negative for infections. She then scheduled me for an ultrasound on the 15th and they said nothing showed up abnormal. I was supposed to start my period on the 22nd or 23rd and I didn't so i took 2 tests and both came back negative. Then on the 26th I think i started my period but it only lasted one day. My period usually comes like clockwork and I don't understand what is going on with me. Could I be pregnant and it is just too early to see on the test?