Paranoid or pregnant?

Ok, I'm on the pill, I had full on friday 9th november and started having period like pains on thursday 15th november. These pains have continued on and off since. Could this be the start of pregnancy? I'm due on my period on thursday 29th november. I took two tests today (4 days before period is due) and they were both negative. I've been panicking ever since the pains started and have been doing lots of reading on the internet, I don't know if its all in my mind and I THINK I've got the pregnancy symptoms I've read about eg. SEEM darker, but I don't know if they actually are, (I never really looked at them before!) I have had breakthrough bleeding and pains in the past when I've been on the pill and so my doc put me on a different one. How soon after would conception occur? And then how long would it take to start getting symptoms? Please help!