'Knee problems'

Can anyone help me ?.

I am 42 female and reasonably fit. I swim 3 times a week about 10,000m a week and compete, I do areobics once a week and most importantly I have recently discovered the joy of munro bagging. But it is the last sport which is causing me problems. On the descent back I am repeatedly bothered by intense pain on the outside of my knee, not the knee joint as I know it but the what appears to be the joint from the side. It only hurts on the outside and only occurs when the walk down is rocky and as I lift my knee to bend for the step down the pain occurs before i make contact with the ground. Going down is no problem when the ground is even no matter how steep and as soon as I get onto flat ground it goes. I have no pain on the ascent and that involves the same movement. I get no pain at aerobics which is high impact nor at swimming when I do all strokes.Only when i have to constantly step over stones and down the way!!!

Please can anyone help me – is all I need to do is the strengthen the muscles round the joint ??

I do intent to pay the doctor a visit but some input would be helpful as I have no idea what could be wrong and I dread to think I may have to curtail my pastime.

Linsey