Hi
Welcome to the MedHelp forum!
A heat sensation in the lower back could be due to pinched lumbar or sacral spinal nerve roots. It could also be due to muscle strain, herniated disc or even due to atherosclerosis of the blood vessels of the region. Sweat dermatitis too can cause a similar sensation. Heat sensation could be the first step of a nerve irritation as in pinched nerves or peripheral neuropathy as in diabetes etc. It is important to get this evaluated immediately.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!