What Causes Dents In The Middle Of Knee Caps?

Since the birth of my son, 2 years ago, ive developed dents in the middle of my knee caps. At first there was one on either side which were about finger width, my doctor couldnt find an explanation and said it was nothing to worry about, they have since gone to 2 finger width, i have pins and needles most the time at the joins in the back of my legs and discomfort when standing for long. I havent had an accident or leg trauma. It looks ugly and is getting worse, what could it bé?