Hello,
You are correct, diarrhea is not a very common complication from surgery and I can't think of a surgical complication that would cause diarrhea 6 months after surgery. One of the most common side effects from Prandin is diarrhea. If this was a new medication around the time of surgery, it may be the culprit. Atenolol and enalapril do not cause diarrhea, it is rare with lovastatin.
I would go to your primary care doctor to discuss options. My guess is that it is probably the prandid.
I hope this helps.