When all investigations are normal, probably you might be having some chronic digestive ailment. You should change your lifestyle to be relieved from the symptoms.
Do regular exercises or even walking about 4 kms./day. Avoid spicy food, oily food, other fatty food items(junk food),take more fibers in your diet in the form of salads, cucumber, vegetables and fruits.
Relaxation exercises, meditation
also will help you a lot.
Wish you healthy life.