What Causes Recurring Throbbing Pain In The Left Knee?

In my left knee only I feel a sharp pain that throbes. It s been going on for about a month now. No Injury that I know of. No redness no swelling. All internal. If I pull my leg back like if I was going to stretch for a run it hurts when I do it. It s not all day, just different parts of the day. I can be walking or sitting and I get the same sensation. Help if anything rings a bell. Thanks in advance