Plz help

So me and my guy friend were getting it on and when we finished he told me the condom broke... We went and got a plan b and my step mom told me I should be getting and early period Bc I took it and I haven't gotten it yet and it's been almost a week since this happened... I'm only 17 and I can't be getting pregnant!! Also when I took the pill it desolved before it went down, is that bad??