You may have a ganglion cyst or lipoma (fatty cyst). As the cyst grows, it can adhere to the surrounding tissue and become less mobile.
I would recommend going to a podiatrist to have the lesion aspirated and sent to a lab for identification and to rule out malignancy.
I had biopsy as requested by rheumy--surgeon took five lumps--one was skin cancer--others were supposedly to be considered fatty cysts, but he said had 'blood supply' & requested to seek w/good dermo--have yet to see already? Could you explain what this might be?? They were in thigh, one on shin, one under elbow on 'chunky' part of arm. I am mid 50's, non smoker...not overweight but seem to be all of a sudden showing my age, & NOT too graceful. Thanks to you for consideration to respond & make in layman terms to understand better. :-)