The root stump seems to have got infected and hence you have got swollen gingiva with tenderness. Further you must be having a third molar erupting which causes some pain in the last molar region. The drugs taken for tonsilitis will have effect on your swollen gum area too. But you might require to take one or two drugs additionally as it may be required.
Next Steps
Visit your dentist as early as possible, take the necessary drugs, remove root stumps, replace it with a fixed prosthesis, then if it's required remove your third molar too.
Health Tips
Early intervention is always better, because having a root stump at this young age shows poor oral hygiene. So check all your teeth, maintain a good oral hygiene. Happy and healthy smiles.