Knee pain on walking down stairs can arise due to early osteoarthritis or even
vitamin D deficiency most commonly.Instability in the knee due to tear of soft tissues within the knee can also give rise to pain, giving way feeling and sometimes sudden locking of knee. Get your vitamin D and
uric acid levels checked fasting and get an x ray of the knee AP, lateral and skyline views. Consult an orhopaedic surgeon in your area with these reports. Also start quadriceps strengthening exercises in consultation with a physiotherapist.