I started taking birth control (desogen) about five months ago, and at first I thought I was just spotting during the active pills, but now I am realizing that what I thought was spotting is actually my period happening.

It is quite weird, the last 3 or 4 days of my active pills, my period will start. It is typically extremely light, like spotting, but is heavy enough to require like protection with a pad or tampon. Once my placebo pills start though, my period will stop.

My period not on birth control is extremely irregular and was always a surprise as to when I would get it, so I started birth control to regulate it. It is now regulated, but it just happens when it isn't supposed to.

Is there any explanation at all as to why my period doesn't happen on the placebo pills? It has been five months, so my body should be adjusted to it by now.