What Causes Severe Knee Pain During My Periods?

I am embarrassed to ask this question, but I just have to find out what it going on in my body. My knees hurt unbearably bad when I have my menstrual cycle. I have tried several different pain relievers and nothing seems to help me. I hurt so badly that I can barely stand up and this has been happening for about 3 months. I am 34 years old.