How can you have a tactile hallucination?

Touched: Tactile hallucinations are bodily sensations when no source for the experience is present. Drug use, particularly with stimulants, can be a cause of tactile hallucinations, especially on the skin. People who have lost limbs can also experience "phantom" pain or other sensations when the limb is not present. In fact, the majority of individuals who have lost limbs experience some phantom sensation.

Hypothesis : One theory that could apply to your question is what's known as the dopamine hypothesis.Dopamine is a nerotransmitter which, according to the theory, is in excess. That means there's too much dopamine in the neuroreceptors, and this contributes to hallucinations.