Hi ! From the history, it seems that you might be having a incisional hernia
which is not very uncommon after a major abdominal surgery
. To be sure if it is a hernia or any other intra abdominal pathology felt superficially, you have to get yourself examined by a general surgeon
in your locality, who may also suggest you for an ultrasound /CT scan if need be to arrive at a diagnosis. The developing hernia can give rise to such aches in your abdomen/pelvis due to stretching of the peritoneum
So far as the lump behind the left knee is concerned, it is unlikely to be related to your abdominal problem. This could be an independent entity like a popliteal cyst
which has become inflamed giving rise to pain and the pain in the foot possibly is referred due to this. This possibly is a benign condition not to be worried about. However, you have to consult your surgeon for this pathology of your knee as well.
Wishing you an early recovery.