I had a miscarriage 6weeks ago and I was 8weeks pregnant.
I went to the hospital 2weeks ago and the midwife said it had been complete, my womb walls were thick again and she said I could be pregnant again. I haven't had a period so far so could I really be pregnant again?

It's possible. You will ovulate before a period so you could have gotten pregnant without ever seeing your period return. Did the midwife order a pregnancy test? You could take a urine test on your own and see what results you get.