What Does An Early Period Indicate?

Hi. I had unprotected sex with my husband on Dec 30th. We went and got the Plan B pill the afternoon of the Dec 31st. I started bleeding January 4th-9th like a regular period even though I'm not due for my next cycle until January 20th. But now I am 6 days late and I have been having pregnancy like symptoms. I have taken 2 pregnancy and both have come up negative. I've been to my doctor and he seems to think that everything is fine but I clearly don't feel like it is. What should I do?