Doctor/nurse can't find my baby..

So I went to the doctor Last week and the doctor couldn't see the baby but did find the gestational sac. (All my pregnancy hormones are "normal.") So I went and did blood tests to find out how far i am. When I went to the doctor yesterday 9/14, the nurse took another ultrasound and couldn't find the baby either, my sac (to me) looked larger than before. But the nurse then turned off the machine and said "I'm sorry." She then went on to tell me my blood type is rh negative and my fiancé possibly has positive which seemed to be her reason for why I will be miscarrying. She never told me how far along my blood test said I was. And she already was so set on having me go to the hospital and remove the sac. But she never said why I would be miscarrying. Nor what type of pregnancy I have. (I.e. ectopic, tubular, etc.) This is/would be my first pregnancy... I guess I'm just trying to figure out if her explanation about my blood is reliable or if I should get a different opinion... so thank you for any replies to help put me at ease..