I am scared...?

My sister had a miscarriage twice... one at 21 weeks due to an incompetent cervix and the other due to ectopic pregnancy. I am 21 weeks along and my doctor said that an incompetent cervix and ectopic pregnancies are not hereditary. I have been cramping everyday, but I'm sure it's just from everything stretching and growing. I don't know if it's because I am at 21 weeks also, but all of sudden I'm scared... I even had bad dreams about it last night. I had a dream that I miscarried because I dilated at 21 weeks. I woke up and I was just freaked out... I know that it's only a dream, but I don't know how to not be scared.