Please help my knee lump

Patient: Hi I recently went to centre parts and whilst there I began feeling a lot of pain in my knee when walking to the point where I could not walk or weight bear. This resolved with rest but now I have developed a hard egg sized painful lump at the bottom of my knee on the outside. I have also started getting stabbing pains in my knee. Please have you any idea what it is. I have not injured it and I am slightly overweight. I am 33 with two children.