No baby or sac found in uterus!?

So ive taken many pregnancy tests and they were all positives. The doctor predicted that im 7 weeks along. I went to do two ultrasound, the gel one and one vaginally, but my uterus was empty! The doctor says I am pregnant, but she doesnt see any baby. She says i might be too early. And ecoptic pregnancy was negative. She also said i might still have pregnancy hormones from my first. But, ive tried for 7mths, all the pregnancy tests were negatives. Until now, i started getting positives. I did a blood test and the doctor says i do have pregnancy hormones, but she wants me to come back to do another one. Ive also experienced an on and off pelvic pain, one sided. And have sore breasts, cravings, just never experienced naseua for morning sickness. What could it be? Im so scared and sad it might be a miscarriaged or a fake pregnancy.