My dad has a sharp pain on the right lower part of his abdomen he had diarrhea the constipation now diarrhea again for three days what kan it be?

Many possibilities. Such a pattern can result from many causes, including but not limited to gastroenteritis, irritable bowel syndrome (ibs), inflammatory bowel diseases like crohns or ulcerative colitis, or even constipation with stool leakage, to name a few more common ones. Evaluation with a good history and examination is needed to pinpoint a more specific cause.