I've never heard of weight loss associated with weed. Pot generally makes most people complacent, laid back, less motivated, and as has been said, more prone to munchie attacks. Legalities aside, I don't personally believe weed is bad for you, if used in moderation. Toking up every day is not my idea of moderation. It should only be a once in a while thing.
As for the weight loss itself, definitely go to a doctor. That's not something you want to be putting off. Don't mention the weed unless he asks you. Why invite trouble?