Missed period?!?!

Hey guys.. so my last period I had was at the end of December. I have two kids my youngest is 8 months old. Is it common for my mentrual cycle to be irregular this late after having a kid? I took two pregnancy tests a week into February and they came back negative. I plan on buying more today. If I don’t receive my period by the end of this month and the tests come back negative as well, I will be making an appointment with my gynecologist. But I’m just curious if anybody has any clue as to why this may be happening? Or if anybody experienced this and ended up being pregnant.