Could I Be Pregnant?

My November period was the 4th-10-th. I then had sex before, during, and after my fertile window. My period was late for December, but the 20th-23rd, I had very, very light bleeding. It was never enough to fill a pad or a tampon in the whole day. I have taken two test and one was negative and the other was defective and didn't have anything in the control window. Is it possible that I could be pregnant?