I was 16 days late on my period what does that mean?

Pregnant or stressed: You may just have some standard irregularity from stress, illness, change in medication, or any manner of mental or physical conditions which may affect you overall. If the entire rest of your life absolutely unchanged, then always consider the possibility that you could be pregnant. Look for enlarged/tender breasts, weight gain, and nausea/morning sickness...

Anovulatory bleeding: Pregnancy is always on the list of possibilities when a woman has abnormal uterine bleeding or cycles. You may be having anovulatory cycles (cycle without producing an egg from the ovary). If a urine pregnancy test is negative, i would recommend evaluation by your gynecologist, and treatment will depend on the cause of the abnormal bleeding and your current short-term pregnancy goals.