What Could A Lump On Knee With Difficulty While Bending?

Hi, may I answer your health queries right now ? Please type your query here... When I sit down I get

a lump in the back of my left knee. If I stoop over I can get it to go away. The knee feels like there is pressure in it. It doesn't hurt but I have difficulty bending my knee. Should I wait a week or so before i see a doctor. I have been applying ice.