Diarrhea is under control after brat diet, but now feel sharp pain in lower back and left side of the stomach, is it due to constipation ?

Unlikely: From your history, you just got over diarrhea, so it is unlikely that your current symptoms are now due to constipation. More likely your symptoms are related to the same virus/infection that was causing your diarrhea or from introducing food too quickly after a diarrheal illness. Before eating a regular diet, continue with bland easy to digest non oily foods. Yogurt is helpful. Soup, rice etc