What causes a person to have a "fear of sex"?

Something I discovered after my first time being “sexual” with a boyfriend is that I have this great fear of sex, especially the penetration part of it. It doesn’t even make sense to me. It’s been suggested to me that I have something called “vaginismus”. Can any of you relate? What can I do to make this better for myself and my current boyfriend? I don’t want this to mess up anymore of my relationships. It’s so frustrating that other people seem to be able to have sex so easily, and I just . . . can’t.