My DD 1 had something similar to this happen to her when she was almost 3. She spent almost a week in hospital as they tried to work out what was wrong. She had numerous tests done including testing for tuberculosis. In the end it worked out to be an infection from a bee sting she'd received a week or so previously. The infection had gone into her Lymph node. She had IV anti biotics and was fine. Lymph nodes can inflame before or after basic illnesses too, such as colds & flus.
I hope everything is fine for your little one.