irregular periods... std or pregnant?

So I'm not on any contraception and recently my periods have been coming so irregularly. They've always been irregular, but I've had 3 this month! They can be 4 days in between or 8 days in between like this week. It's annoying me obviously, but scaring me. I know it's a sign of being pregnant or having an STD... I don't know what to do?! Help!