Help please

Ana

I supposed to be 7 weeks today and I started bleeding yesterday like my period. I have like 5 seconds cramps twice a day or so. I went to a&e and had an ultrasound and they said they only can see the sac which measures 5 weeks? Has this happened to anyone? I was tracking my ovulation and I got positive opk 6 of February which means I should be around 7 weeks now. Doe la this means I am miscarrying?