Unexplained knee pain

Patient: Im 27 years old and only slim but for the past 3 days I have been suffering with extreme pain in both my knees. It hurts to walk imparticularly up and down stairs and to bend or sitIt has been gettin worse and today they are sore to touchI havent injured them in anywayIve tried pain killers and muscle and joint rub but the pain wont easeDo you know what may be causing this?And what I can do?