My knee hurts when I put pressure on it or keep my leg straight?

For some reason, yesterday out of the blue my left knee started to hurt like crazy when I put even the slightest amount of pressure on it and when I tried to stretch out my leg. So in order to avoid it hurting, I had to limp everywhere on one foot and keep my left leg slightly bent. When it's hurting like this, it kinda feels like it dislocated, or my kneecap is off and there's a small space in my knee.

It got better after an hour after I sat down, but it still hurts a bit when I walk around. Even when I was sleeping I had to keep my left leg slightly bent so it wouldn't hurt.

Mind you, I'm not an athletic person. I barely exercise at all other than going on occasional walks to go and buy snacks or something. I usually sit around all day, on the computer other than when I go to help out at my family's restaurant two days a week where I'm standing for 2-3 hours.

Does anyone know what's wrong with me? I've never dislocated my knee before, I haven't got any recent injuries to my leg other than the occasional bruise, I don't have any body disorders that I know of, and I haven't ever broken a bone either.