I usually have regular periods but my period is about two weeks late and my pregnancy tests are negative. why is this happening?

Late menses: 2 weeks late is not unusual for a healthy woman in their life. Stress may be the cause. Repeat pregnancy test if necessary.

Late period: 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). Pregnancy is #1 reason.