An endocrinologist deals with hormones and the endocrine system.. does he think I have a thyroid problem or something? And can anyone who's been to an endocrinologist tell me what happens when you go there?

Could be your thyroid, or a problem with one of your other glands. The endocrine system is very complex and each hormone can affect another hormone, so it's good that you are going to be evaluated by a specialist.
Be glad your doctor is not one of those that will just write a prescription for drugs without first investigating the reason for your weight issue. Smart guy!

that's what I'd guess! An endo will check your Thyroid to make sure your metabolism is ok.