It sounds like you may have an irritable meniscus or medial ligament strain. Just because yu dont notice any swelling doesnot mean there is not an inflamitory reaction taking place, which would explain why it is worse after you have been seated for a while. A meniscus pathology would also fit with your complaint of discomfort at end of range flexion and extension of the knee. Has it locked or given way? and is the pain worse going up or down the stairs, if at all?