I would see a dietitian first.

One of the things I've learned is that I have often been better served by going to some "lesser" specialist first, rather than going to a physician. Physicians know more in the broad sense but often times will have you going through all sorts of tests and things, taking you down who knows what kind of tortuous path only to end up with some simple solution in the end. People who have "lesser" educations generally have more knowledge about their specific little area of expertise than a physician will.

So what I am suggesting is ruling out diet problems with someone who is a diet expert. If that doesn't do it, then by all means go see a medical doctor.