Mystery Pregnancy?! Please help ...?

I stopped taking the pill after I had my last period. I have since then been having unprotected sex with my partner and was due to come on my period 1 week last friday ... still a no show. I have had 3 pregancy tests and all came out negative, yet I am getting a faint line from my belli button downwards (which shows in pregnant women). Has anyone got any ideas on what may be the problem or how to find it out? Thankyou!!