No sex, no pill, no pregnancy = irregular period?

Basically.. at the end of June I had a pregnancy scare. I wasn't sure if any semen had gotten in even though my boyfriend and I didn't have sex. I was expecting my period about a week later, and it came 4 days later than expected (on the 6th of july). It was my normal period. Exactly 3 weeks later (on the 27th) I noticed I was getting another one. My regular period is usually alot longer... about a month and a week.. around 40 days or so. I'm not on the pill or anything so what could be throwing it off this much? It's never happened before!
Please help... and thank you in advance to those who offer any advice.