What Causes Joint Pain After Flu?

I had the flu over new year's weekend including diareah, muscle aches, joint pain, fatigue and fever. The main symptoms went on for 4 days. I was still easily tired for a couple more days after. Since then I have been easily tired with joint and muscle pain off and on. The joint pain is now keeping me down. I'm wondering if this is just reminents of the flu or something else.