I found out I'm pregnant last week. I have been on the pill since I had my second child in June 2009. The 1st day of my last period was between 1st / 3rd October but half way through my September pack of pills I had irregular bleeding for 2 / 3 days it was more brown that red but could I be further gone than I think because I feel bigger that 5 / 6 weeks gone

Possible, but they will go with your last "normal" menstrual period for now. At some point you will get an ultrasound done and that may or may not change things a bit.
It is not uncommon to feel more pregnant after the first since your body is already used to the experience.