I got my period two weeks after my last period ended. what could be causing this irregularity?

Too many reasons...: There are too many factors to list here that could cause irregular periods. You should make an appointment with your gynecologist.

Many things...: This can happen in a normal situation once a year or so. I see though that you are 40 years old, and the likelihood of this happening increases as one ages due to hormonal fluctuations in the so-called perimenopause.