This is called "depersonalization." It is a dissociative disorder that can be triggered by trauma (this often happens during or after a rape, for example), but it can also occur for no apparent reason. (By the way, I am assuming no drugs or alcohol are involved in this situtation - many cause this, specifically marijuana).
Depersonalization Disorder isn't well understood, to be honest, but neuroscientists think that it has something to do with an imbalance in the brain chemsitry. Several things may be of help:
1) Psychological counseling. This helps you understand why depersonalization occurs and trains you to stop worrying about the symptoms so that they go away.
2) Medications. While there are no medications specifically approved to treat depersonalization disorder, a number of medications generally used to treat depression and anxiety may help. Some examples that have been shown to relieve symptoms include fluoxetine (Prozac), clomipramine (Anafranil) and clonazepam (Klonopin).
Good luck and let me know if you have any more questions.