Hi, i need some advice...

i had sex in august (well startied to, but we stopped b4 he ejaculated)... I GOT MY PERIOD IN SEPTEMBER, but i did not get in last month (october) and it hasnt come yet (16th). My period is not usually regular and has skipped two months for the yr (may and july)...i am not experiencing any symptoms (nausea, frequent urination, backache, headache, sore breasts, fatigue...)...but i am still a little stressed about not gettin my period last month.
Should i be worried???
what r the chances of me being pregnant considering the above???
Please help me