Im scared to be by myself, Whats my problem?

Whenever Im alone in my house, I hate it and I cant stand it. We have a 3 story house and I live on the top floor. Whenever my mom and grandmother are gone, I stay locked up in my room afraid to go down to the lower floors. Whenever someone walks on the floor below me, it makes a BOOM BOOM noise every time someone steps. Sometimes I hear this and it really frightens me. We just moved a few months ago from a trailer. I was never really scared in the trailer because I could see every room in the house by just looking straight down the hall. In this house, I cant see every room and that scares me, because I dont know whats in it. I know this might seem a bit ridiculous because I am 18 years old, but I just cant be alone. I guess its a fear that I have. Is this normal?