I am 16 days late and i've taken 3 hpts and they've all been negative and i'm not under any stress. what could be causing it to be late?

Different causes: Multiple causes including stress, hormone issues and still would be sure by a blood pregnancy test.

Causes?: 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.