am I pregnant

Okay so last month a week or two  before my period I had sex and the condom broke then I got plan b. A week later got my period and it was very heavy and horrible cramps. And now I'm in the next month and glow said I should start my period today does that mean I'm late or is glow not 100% reliable or could it come later in the month I'm really scared right now.  I should also mention my period is kinda not normal sometimes I have it at the beginning then at the middle of the month.