What Causes Numbness In The Lips?

For the last 9 months, I have been experiencing a numbness inside my upper and lower lips that feels like a "bunching" of the skin when I pucker my lips. It almost feels like I have been eating alot of salted pumpkin or sunflower seeds. My ENT, Dentist and General Practitioner have no clue what this could be. I tried changing toothepaste, gum, mouthwash and not wearing my nightime biteguard, but none of that stopped the symptom. I have no visible abnormalities. Any idea what this may be?