Why does my mouth twitch when i tap a certain spot on my cheek?

Normal reflex: Tapping certain areas can cause a reflexive movement of the adjacent muscle. This is normal.

Nerve stimulation: You are taping the nerve which is supplying the muscles around the mouth. The response can be exaggerated if you have electrolyte abnormality.

?irritability: You may have Cvostek's sign, which is an indication of low bodily levels of ionized calcium. Simple blood test will help.