You've already gotten some good info from the other responders here, and it's true that burning sensations in the mouth can have a lot of possible etiologies.

I wouldn't dismiss the possibility of Sjogren's just yet with a negative blood test--seronegative Sjogren's is very common. Have you been evaluated according to the diagnostic protocol:

http://www.neuro.wustl.edu/NEUROMUSC...n.html#sjogren

Also, see if any of this looks familiar:

http://www.neuro.wustl.edu/NEUROMUSC...html#burnmouth