12days late, not pregnant?

I've been very regimental when it comes to my time of the month, it has always come on the first few days of the month without a doubt. I have been on the pill for the past 2 years. During September month I missed one day of the pill but had unprotected sex on that day with my boyfriend. With no show of my period this month I've taken 2 tests that both come up negative. I worked out that I would only be 4 weeks pregnant. Could this be too early to tell? I haven't had any symptoms, but again, could be too early?