What does it mean if your period is unusually short; like 1 or 2 days; when you know you are not pregnant? Is there any other reason for that?

Sometimes birth control can make you periods really short. Or maybe it was spotting. Last Friday I thought my period was starting. By Saturday night it had stopped and I thought it was just a short, light period. Then yesterday I started bleeding again.

Oh OK, well I do not take birth control