I have a white patch under my tung and behind my lower teeth. it has been very painful for about 5 days now. i chew 1/2 can of tobacco a day?

Needs examination: Without actually looking at the lesion or your teeth, it is impossible to tell exactly what it is. You need to have it examined and diagnosed by an oral surgeon, ESPECIALLY with your history of tobacco use. The phone call you make to a local oral surgeon for an appointment after reading this answer might just save your life. Please don't delay.

Oral examination: Should be evaluated by your dentist, may need to be biopsied. Once diagnosis has been made it can be treated.

Leukoplakia see DDS: The white patch. It is often caused by chronic irritation or infection, but can also be a cancerous or precancerous lesion especially under the tongue. If you are a smokeless tobacco user, use this opportunity quitting.

Biopsy: See a dentist or oral surgeon or even ENT... it needs to be biopsied for exact diagnosis. I found a lesion like this in one of my patients 2 weeks ago.