Transvaginal ultrasound, baby no show

Trish • 23
I had appointment yesterday ( Saturday ) after getting 4 positive pregnancy tests, im suppose to be 4 weeks but I have this feeling I ovulated late. Anyways the doctor said she didn't see anything on the ultrasound despite a positive urine test :( she even said it might be an ectopic pregnancy ! Very worried.... has this ever happened before and everything went fine ?