A horsefly bite me yesterday afternoon behind my left knee while taking a break from riding a bike.  Never noticed any after pain for about 9 hours when noticed large lump behind knee.  Then slowly felt knee discomfort that ended up getting worse and keeping me up most of the night.  The pain was generalized at the knee but radiated up and down my leg.   Figured I would go to clinic in morning, but pain seemed to get better as day went along.  Now that its bed time, pain is back in knee ready to make my night another great one.  Now question is, is this something I should be concerned about? or something that just needs a couple days to get better?
Welcome to Chat Doctor. 1. It could be infectious process as lump with painful condition, source of chances of bite can be possible - but it's not poisonous as no other signs are seen.2. Other possible causes I can think of is ganglion, cyst arising from knee joint.3. Need to examine, if infective a course of antibiotics will help, in larger one a small incision and Chat Doctor. 4. As a remedy - do not squeeze, scratch the lump as in that way you will complicate the healing process.5. Maintain local hygiene, clean area with antiseptic liquids. Good luck. Hope I have answered your query.Anything to ask do not hesitate. Thank you.