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.