Hi Dr. I just want to find out why this is. From yesterday I got pain on the tight side under my ribs. It only hurts when I walk.

Not sure. Without an exam and history it is tough to truly diagnose you. With no apparent injury, you may have a simple muscle strain. If your symptoms worsen, see the Dr.
Depends. I don't know how painful it is or other associated symptoms such as fever, recent injury, other recent illnesses, past surgeries, it could range from mild muscle injury or rib inflammation to kidney stone, appendicitis, gallbladder or liver issues. Best to call or go to your doctor today to get his/her advice or go to urgent care/ ER for evaluation.