Hi Doctor, My 13 Year Old Lab/pit Is Not Feeling

Hi Doctor, My 13 year old Lab/pit is not feeling to good. She always has been a energetic curios dog. She is over weight. Ever since she was a puppy she has LOVED to eat. She loves to take walks or play ball. About 6 months ago I noticed a cyst on her back leg inner thigh. It seems like it is full of fluid. It doesn't seem to bother her to much. She always has pep in her step. Until Yesterday morning. She is very lethargic and looks very sad and sick. She had no problem eating her dinner 1 hour ago. Is there anything I can give her to help her feel better?????
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