What Causes Delayed Periods?

I had my period august 3 and it ended on the 9 and then on the 19th I had sex and I'm not on birth control but I did use a condom and in pretty positive nothing happened to it and it was fine and then I got my period again on august 26 through the 31 which is syrange cause ive never had it two times in one month and now it's the end of September and I haven't had my period and I'm really scared I'm pregnet and I don't wanna be cause I'm only 17! Please help!