Is Everything Okay With Me?

Hello, a virgin girl of 18 writes to you. I have a friend with whom I still don't have sex, but I want to .. the problem is that every time we push each other I get aroused for a very short time, I get to some phase of arousal and suddenly everything ends and I am forced to moan pretend to good for my friend. He does his job well, the problem is in me, because when I try to masturbate the same thing happens ... I don't know what is the reason, am I sick, I feel bad and a little scared .. I ask for advice if anyone knows what due to write!