I have frequent bowel movements, two and sometimes three, at times loose motions. I am not gaining weight even though I eat well. I do not eat fatty foods, but vegetables, rice, curd, milk etc.
You might be lactose intolerant....it is difficult but do not have milk or curd or any dairy products for a couple of weeks and see if it makes a difference...For milk, you get something called Lactaid, which is milk without lactose...you can try that as a replacement for milk.