I would call your physician. Make an appointment for another ultrasound. They may need to draw blood for a quantitative HCG level (pregnancy hormone). From these tests your physician should be able to tell you.
I'm not sure why you got the negative result maybe the urine concentration was too weak? was the bleed just a small amount? Not like a normal period type bleed. Bleeding in pregnancy is quite common and as long as it wasn't very heavy with clots then you should be ok. If you have had a positive pregnancy test since and are gaining weight as you would expect by this point then at a guess I would say yes you are still pregnant.
Have you had an ultrasound yet or scheduled to have one soon?