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?