Is this bleeding normal?

Between the 28th of March and the 2nd of April I was on my period. On the 6th of April I had unprotected sex. On the 7th I went to the school health nurse and took the morning after pill. I didn't have any side affects after I took it. It's now the 12th of April, and I found blood in my underwear? Obviously my period is not due anytime soon, but I'm in my fertile days. I never bleed before or after my periods. It must be the pill so is this a normal side affect? I'm unsure since it has been 5 days since I took the pill. Please help! :)