Pain in rectum, lower back, vomiting & diarrhea?

stomach upset: You are describing acute gastroenteritis it will eventually subside and it is self limiting.

Diarrhea/vomiting: Gastroenteritis is a term that describes either a bacterial or viral infection of the intestinal system that can cause symptoms of diarrhea and/or vomiting. Other symptoms may be low back pain and rectal discomfort due to diarrhea. Worsening symptoms require follow up by a physician.