positive test but no pregnancy on ultrasound

I went to the doctor today because I was experiencing some bleeding and passing small clots. according to my last period I should be around 6w1d. I was given both urine and blood tests and they both came back positive but when they did the ultrasound nothing showed up. I have to go back in 48 hours to see if my blood levels are increasing or decreasing. I'm worried I might have miscarried or maybe it's just too early to detect anything. has anyone else been through something like this? I would appreciate some advice.