hey, as per your complaints, I can say, you are having sensitivity because the filling has come out and so now your dentin is open to saliva and food. which is causing sensitivity. second the other side, if there is swelling then its definitely an infection. the caries of your teeth has caused infection in bone and gums. its called periapical abscess

U have said sensitivity, but there is a swelling on ur cheeks. It might be infected and u will definitely be having pain. Try eating something with that tooth and check for any pain. Possible treatment shall be a ROOT CANAL THERAPY

The filling which has come out needs to refill as early as possible , and the swelling on the opposite side need to check clinically and small X-ray to check if cavity has reached nerve and plan the treatment