Stopping the pill?

I stopepd taking the pill midway through the pack on the 18th may and had my period on the 22nd to 25th of May....we've had before the 22nd and after the 25th and I know not to take a pregnanacy test until and if I have missed my next period....when will that be if i didn't finish the full pack of pills? will it be 28days after the first day of my last period? or have I got it wrong? Put me straight someone please :-)