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This swelling is most likely a Lymph Node. Lymph Nodes are normal body glands which can get enlarged by a lot of stimuli like infection or inflammation. This is a normal response. Most likely that swelling is due to Lymph Node, which is due to an infection or inflammation in the throat / neck (like a sore throat / tonsillitis). For the time being you can observe the swelling for 2-3 weeks. Most likely it will subside by its own by that time. If not you can go your GP for a neck examination. (To see whether some other nodes are also enlarged / also to see whether there is any foci of infection or inflammation).

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