What Causes Large Lump On Right Knee With Tightness In Glutes?

Hi, I am a a very fit and active lady in my early fifties and over the course of the last two months have a egg sized lump on the outside of my right knee. I have been to my GP and had an x-ray. This confirmed that it was not calcified. However, it is still there and causing discomfort when I walk and or bend. I used to run 4 times a week, but have stopped since the onset of this condition. I am also feeling a tightness in my glutes which tends to want to cause a twisting motion when I walk. Any thoughts on what this is and what can be done about it? Many thanks