Severe Knee pain

Hope someone can help with this.
Problem is a Sudden onset of knee pain when walking to the point that I can only walk about 200m before having to pull up. No pain when sitting bending, or going up down stairs getting up moving from side to side, but as soon as I start walking the pain builds to point that I have to stop and this takes as I said about 200m. No history of knee pain before this and no obvious injury happened. any ideas anyone?