It depends the kind of test they did. If they did a qualitative rather then quantitative the threshold for HCG to be considered positive could of been higher then what you're levels were at the time, while the threshold for what would cause the urine test to be positive was lower. The fact that the ultrasound looked good is great. Everything is probably fine I would just request your Dr give you a qualitative HCG then again 48 hours after the first to check that you're hormones are rideing appropriately. They should double every 48 hours.