What Causes Stomach Pain And Bloating Post Full Knee Replacement?

My mother is 83. 10 weeks post full knee replacement. For the last few weeks she has had stomach pains of various intensity. She feels bloated, clothes are tight, and she is four pounds heavier than normal. She has a very set diet and does not often splurge or overeat. Pain is in the middle of the abdomen and does not seem to be related to what she is eating.