My period is irregular & and i didn't have my period last month and i had sex. & it's happened before. but i don't if its stress again. ?

Unprotected sex?: Take a pregnancy test now if your period is late. Reasons for missed / late period include: an anovulatory cycle in which no egg is released, marked v or ^ in weight; eating disorder; over-exercising (endurance athletics), stress, travel, significant illnesses, oral contraceptives ; other meds, illegal drug use, breast feeding, hormone imbalances and medical problems (i.e., pcos).