Suggest Risk With Thick Uterus Lining

Hi! I ve tested 3 pregnancy tests which all were positive. I am almost 6weeks but 4 days ago when I went to get checked up for an ultrasound they could not find a sac. All they saw was a thick uterus lining. Is this normal? Plus I have all the symptoms of being pregnant. I had slight period like cramping. People say that my embryo is implanting itself to my uterus.