Could I be more than 6 weeks pregnant?

I have 2 girls, a 4 yo and an 18 month old. I had been taking birth control pills since I had my last daughter. I had my period from December 1st-7th, then I started my new pill pack and took them for a week when my husband and I decided to try for baby #3. when i stopped my pill pack in the middle it made me start my period again from December 18th-23rd. I had a positive pregnancy test on January 14th. I know that the doctors go by your last period but I was just wondering if it's possible that i got pregnant before or if they should be using my first period since my 2nd one was due to me stopping my pills. (and it's not considered "trying" when you get pregnant the first month..very fertile!!)