Infection etc. The swelling process found is equivalent to inflammation which will take time to resolve.
Inflammation. As your body fights an infection, the tonsils (back of throat) are one of the major trafficking sites for your immune system to recognize and fight off infections. Generally, redness, swelling is a visible sign that the cells in the area are releasing cytokines (chemical communications) to cause your blood vessels to dilate and immune cells to slip through and infiltrate the involved tissues.