What Causes Inconsistencies In Periods?

I just started birth control for the first time. And although I was suppose to start my period before starting the birth control I decided to start it anyway, before having my period. I receieved it just after a couple of days of starting the medicine. But now I've had it for about two weeks. And soon my normal scheduled period is going to start. What does this mean? Is this happening because I started birth control before I needed to or what? It doesn't really bother me, I'm just tired of having to deal with this lasting period. I would just like to know if I need to be concerned or is my body just getting use to the new medication?!?!