What to do if i have the worst cold sore ever. the pain is spreading into my jaw. what is going on?

Herpes infection: Many cold sores are caused by a herpes virus that spreads down a nerve from the spine, they often recur at the same location. When they become severe or occur with increased frequency a trial of an anti-viral can supress the infection and decrease the frequency of recurrent attacks. Triggers for attacks vary from person to person and can include illness, stress, sun exposure and sometimes foods.