How!?!?

I had my tubes tied October 2016. Well 2days ahi I started having stomach pains so I went to urgent care. They did a pregnancy test and it was positive so they sent me to the hospital for an ultrasound due to my tubes being tied. Well turns out I’m 6weeks 1day today. Due December 2019. Not sure how it the heck it happened. Anyone else ever get pregnant with their tubes cut, tied, and burned?