I think you have abscess which is chronic.It might be due to tooth decay or infection in gums.If you have pain from adjacent teeth,which might be continuous and radiating to head,then it's bcoz of tooth decay......if pain is dull aching increases with swelling then it may be due to gum infection.
Next Steps
consult nearby dentist,they may take an xray and give you exact diagnosis and treatment
Health Tips
Take painkillers like Zerodol-sp as an emergency to get relieved from associated pain