my daughter has been feeling very tired, achy, and has had diarrhea every time she goes to the bathroom for about 3 weeks. She can function, and goes to work. but as the day progresses she is very lathargic. sleeps more than normal. not much a fever.
You have not mentioned the age of the child which is very important. The case would different in each of the situation if it is a newborn, 6 month age or 6 years of age. If a diarrhea continues for 3 weeks it may mean some malabsorption. The role of diet and diet change comes into play here. Diarrhea can happen when a semi-solid food or some different food is introduced as in case of a 6-month-old child undergoing weaning. Lethargy is not a good sign. It might either indicate that the child is having deny Chat Doctor.  A blood investigation and the investigation of the stool is also required.