What causes constant sweating, to include hair and feel internally?

Hello. Thank you for posting today.

If heavy sweating has no underlying medical cause, (you would need to see a doctor to find out) it's called primary hyperhidrosis. This type occurs when the nerves responsible for triggering your sweat glands become overactive and call for more perspiration even when it's not needed. Primary hyperhidrosis may be at least partially hereditary.

Helena