Very irregular

[oldflattop.livejournal.com]

Ok so I got my period (in a pretty hilarious and mortifying way) earlier today. But I finished my last period 11 days ago and had finished the one before that 18 days earlier. Wtf? I am not on birth control, but typically have a very regular period and am able to predict when they're going to come. This was totally out of the blue. I haven't noticed any symptoms I usually experience right before I get it and I'm just really confused.

The last time I had sex (which was unprotected) was in the last week of August. Any ideas or suggestions would be greatly appreciated.

update:
Thanks for all the great advice. I just took a pregnancy test with negative results (whew!) and set up an appointment to get back on birth control in two weeks. I recently (as in a month ago) moved across the country and have had a very stressful last couple of weeks. So as of now I'm just writing off these freaky periods as nothing to worry about. Thanks again! You guys are great :)