Dates off

I should be 8 weeks pregnant, last period was January 9th after chemical in September. I tested positive February 10th, had an ultrasound March 1st where I was measuring 5+2 when I should have been 7+2. Could the dates be 2 weeks off?? And if so how did I test positive SO early. I’m worried and have another scan March 15th to see if baby is growing or if beginning of miscarriage. Took these test last night and this morning and still have symptoms so I’m hopeful but worried! Any advice??