My 8 Year Old Son Started With A Fever 4

My 8 year old son started with a fever 4 days ago, the fever lasted about 3 days. He was complaining of abdominal pain. I took him to the urgent care and they diagnosed him with an ear infection. This morning he woke up, no fever but had a hard time standing up and said he couldn't walk. He said the pain is in the back of both knees. Not too sure if this is something I should be worried about.