The most common cause of this is swelling of the lymph glands. This can be caused by an infection or even a bigger problem. Root tooth problems can even cause swelling in the throat and they can be pretty serious.
I suggest that you see the vet tomorrow to have this checked and to discuss your treatment options. This could have some pretty serious possibilities so should not be taXXXXX XXXXXghtly.
Let me know if you still have questions.