Wet sensation behind left knee. Why?

I have recently started getting this cold/wet feeling behind my left knee and sometimes on my stomach but when I go to wipe the water there is nothing there. Lots of people  talking about this but no answers online.I have been struggling a lot over the last year with digestive problems and malabsorption. I had Campylobacter, Helicobacter, Pancreatitis and numberous Closs Diff infections. Now I am suffering and struggling with ongoing abdominal pain, stool changes and post infective IBS. I did have a folate deficiency in January but had a normal result in March so I am no longer taking vitamins. I have constant fatigue and muscle weakness where I get regular cramping in my legs from doing nothing! Recently my immune system has been low enough to give me a chest infection and severe bacterial conjunctivitis. Yet, over the last week I have had a strange sensation that the skin behind my left knee is wet. I am not taking any different medication than usual and do not understand why?