I really need opinions.

Af was due the 20th. I'm a very regular cycle. 28 days 5 days bleeding comes like clock work every month. On the 20th I started spotting, only visible when I wipe. I assumed that af was coming and that I was not pregnant.. Today is the 22 and in still spotting, visible only when I wipe. This is not normal for me at all! Every test I take is negative.. Could I be pregnant and that is the reason I'm only spotting??? But if I am why would the tests come up negative? Please help.