Hello, Welcome to healthcare magic forum. Swelling in the roof of mouth may be due to-deep dental caries or periodontal infection of upper anterior teeth on left side,which has progressed into periapical area causing spread of infection into sinuses as well as tissue spaces. I would suggest you to get a thorough clinical evaluation by a dentist. Pus within the swelling has to be drained completely and curetted. Tooth has to be root canal treated if necessary. If your oral hygiene is poor,get a deep scaling done. Usually in such cases,doxycycline along with analgesics has to be administered. Maintain oral hygiene well. Discontinue use of appliances or dentures causing infection or get them replaced. Hope this helps. Take care.