What Causes Numbness In The Fingers, Tongue, Feet And Lips?
Metabolic alterations most likely cause.
Detailed Answer:
I read your question carefully and I understand your concern.
The fact that your symptoms are so widespread, involving limbs on both sides, tongue and lips is in a way reassuring. That is in the sense that it points against threatening brain lesions such as stroke, tumors etc where symptoms are more localized in certain regions depending on the site of the damage.
Judging from the history of a flu syndrome I would say the most likely cause would be dehydration and changes in electrolyte levels. If the flu is getting better should be transitory with good hydration and nutrition.
Other possibilities would include hormonal alteration such as in the thyroid and parathyroid hormone levels.
If symptoms persist some blood tests such blood count, glucose, liver and kidney function, electrolyte panel, thyroid function are advised.
If new symptoms arise do not hesitate to write again.