It is possible that you have already aborted the fetus and so nothing was seen on ultrasound
test can remain positive for about a month after abortion
But before assuming this as a possibility, it is better you get a quantitative beta HCG
level estimated. This is to rule out ectopic pregnancy (pregnancy outside uterus).
If beta hcG are high enough or show rising trend with no visualization of intrauterine pregnancy on ultrasound then it indicates chances of ectopic pregnancy.
Also at 9 weeks gestation some products of conception should have be seen in uterus if it was a intrauterine pregnancy even if you aborted.
would show falling levels or very low levels.