Lindamh,
Numbness and tingling is a sign of a peripheral nerve being injured. The injury is usually due to compression of a nerve. It is unusual though, for this type of problem to affect just the lower half of the face on both sides. That would require nerves to be compressed on both sides of the face.
It is more usual to see numbness on one side of the face, in the dermatomal pattern of a peripheral nerve, such as the facial nerve or one of the cranial nerves.
However, it is quite common for a patient to have peri-oral (around the mouth) tingling in cases of hyperventilation. In hyperventilation, the mouth and the fingers often tingle or even go numb. When the patient returns to a normal breathing pattern, the tingling goes away.
But, if the tingling persists, the numbness gets worse, or you develop problems moving the lips, you should probably have that looked at. A neurologist would probably the best doctor to see for this.
Good luck.