My period is 16 days late, what could be wrong? i have had unprotected sex, but three weeks ago i had a test done but it was negative, but it was probably to early to tell.

Prego or not: This is not that uncommon. Pregnancy comes to mind but a pregnancy test should be positive if done correctly when a period is missed. It could be too early & a repeat using the first morning urine can be more accurate. But some women occasionally skip cycles. It can go beyond 60 days without pregnancy and at that point a gynecologist might offer treatment to trigger one.