Over the past six months I go to do this certain exercise, I get on my hands and knees and put my right/left arm out and opposite leg. Your supposed to stretch them out and then bring them in and touch elbow to opposite knee and repeat. Well when I try to conect my elbow and knee my the back on my thigh gets this pinching-pop sensation. It's very painful and makes it hard to walk for awhile. I have gained 40 pounds in the past 9 months and my knees are weak. Well now it happens when I walk as well and I have little bruises forming on the back of my knees, (especially my left one). I do yoga stretches, walking and aerobics. I have lost a some weight over the past month but the problem seems to getting worse. I am unable to afford a doctor for a few more months, I was hoping someone knows what it is and what I can do about it. Oh and just in case I should mention that I injured my lower back and have nerve damage and once again will not be able to see a doctor for a few more months. Any advice will be greatly appreciated.