Am I really pregnant?

I found out I was 5 weeks pregnant about 2 weeks ago so now I'm around supposedly 7 weeks. I have not had my period since early-mid september which was why I tested. I was shocked bc my periods are usually irregular anyway. I took a urine pregnancy test at my hmo and it was a positive result, also took it at planned parenthood and they 2 said it was positive but at first they couldn't find my specimen! I'm thinking is it possible they were reading someone else's result? And as for the hmo test could it be a false positive? Then I had my ob-gyn doctor do a l ultrasound with really low tech equipment by the way and she said I was around 5 weeks approx. I saw the "embryo/fetus" on the screen on the bottom far right side of the scan of my womb and it was the size of my fingernail, she said it was the "gestational sac". But couldn't it have been actually a uterine cyst or tumor or something instead? How can doctors differentiate between a uterine cyst/tumor and actual fetus/embryo