5 weeks pregnant and bleeding

So I am supposed to be around 5 weeks pregnant. I took a pregnancy test about a week ago and it showed positive. On Saturday I started heavy bleeding and was worried I may be miscarrying I was getting cramping and blood clots. Today I called the Drs and they told me they will need to arrange and ultrasound for me. First I would need to come in and take another pregnancy test to confirm the pregnancy. So I went and they took the urine sample and it came negative so dr just said looks like I was never pregnant. I'm sure I was. I was late on my period and I had the symptoms. He was adamant and said with it was very early and I miscarried on Saturday or I was never pregnant. I don't know what to think or do now.