What can cause weight gain? i have a low fat healthy diet and exercise (walking) daily. why am i still gaining weight? i am not on any medsication. please help?

There is a Nobel prize waiting for the scientist who can answer this question. Many obese people have no greater calorie intake and no less exercise than many people of normal weight; it's a major scientific mystery. The important thing is to find a doctor -- ideally a metabolic specialist -- who understands obesity management. Ask your GP for a referral. Good luck!