Knee problem: any advice?

i was at rugby training last night and as i moved off to jog midway through the training session, i felt a slight twinge of pain in my knee. it was then painful when i put weight on it when the leg was bent and i had to stop. it is painful when going upstairs for example and my leg is taking weight. the pain is central, from below the kneecap through to what feels is like the inside middle of my knee. does anyone know what this could be, how it is best treated? many thanks!