As I was taking off my pyjamas last night I suddenly felt a stinging pain in one spot on the back of my knee - assumed I was bitten by something, didn't think much of it. Anyway today it's still there, really stings whenever it is touched but there is no mark or bite whatsoever that I can see - not red or anything. What could it be?! It feels a bit like a wasp or bee sting but with no visible evidence of anything!