Three negative home prgnancy tests after unprotected sex. one at 40 days one at 37 days one at 30 days. still very late period. why still late period?

Don't discount: the stress of worrying (it sounds like) that you are pregnant! The hormones that regulate the menstrual cycle, progesterone and estrogen, are constantly in a state of flux and they determine when menstruation starts. Stresses including illness, fatigue, travel, worrying about pregnancy, can all have an affect. Your period should come; 40 days is 12 days late. Pursue reliable contraception, if need