Coeliac disease is a condition where one can not tolerate Gluten, which is present in Wheat, Rye & Barley. It is usually picked in children when they are not thriving well despite a reasonable diet. They may be feeling tired and lethargic all the time. One of my frined’s daughter was picked up to have this as her Vitamin D and Iron levels were not getting better despite adequate treatment.
Luckily nowadays, there is plenty of Gluten free food available in superstores and online. It may take a while (months to years) before the weight gain starts to optimise, specially if it’s a late diagnosis. This is because the villi in the intestine take time to regrow to their normal (ish) size once the gluten remains excluded from the diet.
There are excellent resources online where you can look up symptoms and join the organisations like Coeliac UK.