Could there be a possibility that I am pregnant?

Fellow mommas. So I had a baby in late November. So my six-week mark would have been around the first week of January. My boyfriend and I had sex just short (days) of my six-week mark. Well, I went in on January 21st to get my birth control. My pregnancy test came back negative. But is it possible I could be pregnant, and it was just too early to tell? I don’t necessarily think I am, just a concern of possibility. Thanks!