Can lifting after appendectomy cause internal bleeding?

Probably not. The admonition against heavy lifting for a time after abdominal surgery is out of concern for the surgical wound(s) in the abdominal wall. Your surgeon worries that you might somehow disrupt the wound, either opening the skin or creating an incisional hernia. At the very least, heavy lifting would be uncomfortable soon after abdominal surgery. There should not be a risk of internal bleeding.