There are so many different possible ways that these things that you stated could be leaning towards. As I read further, I saw that you are already scheduling to see a endocrinologist, which was my 1st suggestion. I would say that this is the best route to go at this point.
You did not mention anything about diabetes or any symptoms other than weight gain, but I wanted to add that when you have a child that has excessive thirst, urination, sweating or weight gain to name a few, these all could be signs of diabetes. Before you prepare yourself for the worst..see the endocrinologist.
And of course, if you are not satisfied with what you are being told and still feel uncomfortable, you have every right to get a 2nd opinion.
Good luck :D