Ughhh the doctor has me worried

I took a test on may 20th it's said I was 2-3 weeks pregnant so I made my appointment. I went to my appointment Thursday June 11th and my ultrasound measures out to be 4weeks but my doctor told me that I was either 4 weeks or I was having a miscarriage so now I am waiting for two more weeks to go back for another ultrasound because she said if there is a baby showing then I will be 6 weeks if no baby I'm having a miscarriage now everytime I have a little cramp or a little discharge I get scared but I'm not having any miscarriage symptoms