confused

So I had a normal period from the 14th-20th of November. Had unprotected sex on the 26th then had another period December 22-26th. And haven't had any period symptoms or anything. I had acl and meniscus surgery Jan. 11th and was asked to take a pregnancy test before. Which must of been negative because the doctors said nothing about it while prepping for the surgery the same day. And I still haven't had a period for the month of January. What could be going on?