If it persists, as in your mother's case, she may have what's called a 'peripheral neuropathy'. If the sensation is due to nerve damage, more than 100 causes may underlie the problem. In diagnosing this ailment, her physician will likely have ruled out a superficial irritant or blood vessel disorder. The next stage is to consider a nerve disease and its underlying cause. At this point you should consider consulting a neurologist who is the most likely to come up with a definitive diagnosis and instigate long-term relief.
Peripheral nerves do heal, but only slowly so it can take months for symptoms to subside.
In the meantime, there are some self-help measures to try. Proper treatment will be based on the underlying cause - herbal preparations are not rationale therapy. Best wishes to you and your mother.