How often are you stepping? I've had bum knees since I was 14 and have to be very careful of them. When I have tried to step every day, it bothers me so I do step on alternate days.
How many risers are you using? Have you recently increased them? Have you been doing any impact moves on the step? The steep stairs definately could have caused the problem. Since you just started back stepping, make sure you are starting slowly because injuring your knee can really screw things up.. trust me on this one.
Try either reducing the number of risers, doingt he routine w/o a step even and maybe for a few days not climbign the steep stairs. Through trial and error you should get a good idea of what the culprit is. If this doesn't get better, do go see a doctor.