What Causes Swelling And Pain In Joints Post A Bout Of Fever?

My husband has swollen joints. His first symptoms started about march the second vwith a high fever and flu like symptoms. He has started swelling in joints and has a pitting x4 in his feet. He went to the family doctor he was given a cough syrup. At first I thought it was an allergic reaction he has not taken the cough syrup in 24 hours and swelling is worse with pain in joints they are warm to touch. His vitals are stable an within normal limits