Should I be worried?

So I'm 18 I was put on the injection for contraception at 17 and for me that stopped my period I stopped using it for health reasons as it changed my mood and all that but I finally got my period back 3-2 months ago but this month it hasn't arrived and I'm 15 days late but I know I can't be pregnant as I haven't had sex in 8 months should I be worried or is it okay?