LOST PERIOD. NEED HELP FINDING IT.

Kay
I'm over a week late. 
My "friend" and I did the deed while I was on my period last month. He thought "Well, you're on your period, you can't get pregnant." And "finished" inside. My period stopped the next day. 
Is it true? Can you not get pregnant on your period?
I thought it was possible because the egg is still there. And on top of that, it stopped the next day. 
Should I be worried?
I've taken 2 tests that came back negative... But I don't know if I trust it.