sometimes your parents do it on purpose so that you always try your best in everything you do- my dad constantly tells me to go get something done, like get into uni for example and once i done that tells me to get good grades etc, it does get really annoying and i do get put down sometimes but i know that they are proud of me and the only reason he does it is so that i become the best i possibly can, career wise, health wise and socially. if they don't push me then no-one else will.
but i do see how it has become irritating for you maybe your mother used to be your shape at your age and was like you diet wise etc and sees that you may potentially put on weight like she did, have you looked at it like that? maybe she could do it in a nicer way, but you need to find out her motives first.