Q. Should I be concerned about pain and diarrhea post surgery?

Hi doctor,

My ten-year-old daughter had an appendectomy two days back. She was in horrible pain after that. They wished to send her home, but they decided to keep her in hospital since the pain could not be controlled with Tylenol or Ibuprofen. Then the following day, she had terrible diarrhea. She went around ten times in two hours. It concerned me, but they informed me that the diarrhea was normal. Her pain was still pretty bad, so they finally decided to try something stronger than we could take her home. It did work for three hours, and they sent her home. By the time we got home, she was in horrible pain. She was crying most of the night until falling asleep. Yesterday, I noticed a lump on her back that was warm to the touch, and when I touched it, she said that lump is the reason for her pain, and it hurts her badly. As I was concerned, I called her doctor, but they said that they had no idea what I was talking about and they would like to check her in ER the next day. I know as a parent, we overanalyze everything. I did not take her to the ER because of a muscle strain. Also, I do not want to overlook something that could worsen her. What could the lump be? Should I be concerned? She still has fever and pain. Please help.