Pregnancy after tubal ligation

I am 32 and had a tubal ligation in Dec of 2009. I had my period Dec 24th-28th (Normal for me) then restarted my period Jan 5th-8th. I had a doctor's appointment to address the period as this was very abnormal for me.

My doc ran a pregnancy test out of protocol regardless of my history and Bam: Pregnant.

Ultrasound shows endometrial lining as being 4mm (Not consistent with viable pregnancy, but possible) and no obvious ectopic pregnancy. hCG value of 737. Today my value is 715. Doctor wants to run another hCG test on Tuesday to see where I'm at before proceeding.

Has anyone ever had this happen?! Any and all advice would be greatly appreciated.