Injured knee

So I have no idea what I did, all I can remember is getting a Charlie horse while I was sleeping, but ever since then my knee has been in pain. Not excruciating, but worse than my regular arthritis. There is no swelling, stiffness, heat, excess fluid, and I can bend and extend it all he way with no additional pain. Walking doesn't aggravate it all, but walking down stairs hurts tremendously. I have no medical insurance so I need everyone's ideas on what I did and how to fix it!