What does it mean if you cant stop thinking about your boyfriend?

It's called limerence or rumination. It is fairly common, especially in young people, since it is a psychological state driven by neurochemistry and hormones.

If you were not in a relationship with the object of your romantic obsession, it would be called infatuation.

If it were a pathological and unhealthy obsession to be in a relationship with the person, it would be called erotomania.