What Causes Pain In The Knees While Walking?

About 3 or so days ago I woke up with a pain in my knee, it's only at one part, but it hurts to walk and when I 1st start walking I have a limp, after a while it mostly goes but then as soon as I stop, then start walking again it's back, it started to go away then on my walk it felt uncomfortable and a bit of pain what's wrong with my left knee?