I fell over on gravel about a month ago and got a very large, dirty cut on my knee. Since then it's healed well but has still been painful to kneel down but I've been trying to carry on as normal. I knelt on the floor last night and got a very sharp pain in my knee which lasted around five minutes. About half an hour later I noticed a soft lump which looked like it was full of fluid. I stuck a pin in it to drain it and light yellow fluid came out. Since then it keeps filling up again. I've never had anything like this before. Does it sound normal or do I need to see the nurse tomorrow?