Could I be pregnant?

Daisy
So I take the combined pill, I came off my period exactly a week ago (Saturday 13th Feb) I continued to take the pill after my period, but I missed two pills the following Tuesday and Wednesday, in this time I had sex (obviously unprotected as I had missed my pills) I took the pill again on Thursday but noticed i had bled, I continued to bleed on Friday (took the pill then too), but it wasn't just spotting it was the normal amount of blood I usually get during a period. I am now on the third day of bleeding a week after my period, and its getting a little lighter but i still have bad cramps. Also I thought it was worth mentioning that I have been quite nauseous recently, like I'm actually going to throw up and I have had a few headaches. Could someone offer me an explanation as to why I am bleeding a week after my period? What could be the cause of this?