Birth Control and Cramps?

So, today I took my first birth control pill (this is the first day after my period btw) and immediately after I took it I started cramping a little. It's been at least three hours since then and my cramps are getting worse. Is the cause from my birth control, fertile window, or still from my period? I'm pretty ignorant when it comes to this kind of stuff so I could really use the advice C: thanks ladies!