Hello dear, I am here to help you. No need to worry, majority of the population who got caught with flu suffers from post flu symptoms. Coming to your situation most probably pain in your knee may be the post illness symptoms plus the effect of the workout in a recently ill body.So be relaxed, here I am advising few things • Drink Plenty of water • Dont exercise vigrously for few days as your body is recovering from the previous illness. • Try to eat much of green vegetables and fresh fruits and juices. • Take Rest • If pain is more then you can take analgesic tablets like ibuprofen or acetaminofen thrice daily for 1-2 days. Your symptoms should start subsiding within 1-2 days. Hope this will help you. Thank you Take Care