Suddendly , The Left Side Of Area Below My Ear

suddendly , the left side of area below my ear along jaw line swelled and got hard. After about two hours the swelling went down. I also have mucous in sinus areas and but no other cold symptoms. no fever no runny nose. I have had this happen before and mentioned it to my doctor. He said it could be slivitary gland and as long as it doesnt stay there not to worry.any suggestions as to what might cause it ?