Re: what's wrong with my knee?
Had a similar problem a few years ago. I could do everything without discomfort, except bear direct pressure to the front of my knee, such as during shikko or seiza. MRI revealed minor strain of my patellar tendon (holds the kneecap in place). The injury healed on its own in a couple of weeks.
That said, I hasten to add that your injury may be different or more severe than mine, and if you have had pain for a month, you need to do like these other people have advised, and haul your ass to the doctor. A visit directly to an orthopaedic specialist would probably be the most revealing. Your knees will have to last you a lifetime, and are nothing to be trifled with.