I Went To The Doctor Maybe 3 Weeks Ago Because

I went to the doctor maybe 3 weeks ago because I was sick with fever sore throat and various aches and pains, I was given a shot to decongest and 10 days worth of Cefdinir, all other symptoms are gone but I still have a sore throat that comes and goes and the back underside of my tongue feels sore. Today, I noticed the roof of my mouth is yellowish in color as well as slightly the underside of my tongue. What could it be? I'm really worried