thats way to vague of a description. There are certain tests that pertain to different injuries. If this is just pain from the day after and then goes away, i doubt it a 'formal injury'.
could just be that your not consistent and your knees need to get used to it?
walking down stairs is much harder to do than going up, biomechanically and requires more strength at a certain range of motion, so my guess is that if it continues to hurt, see a physical therapist.
depends on where it hurts in the knee to be honest.. could just be simple patella femoral pain