First, hello Millie!!
ok, first part.. you're going to see a Peds doc... for..? Is he supposed to see you for HRT, a checkup (you didn't say your age, tho if its Peds, guessing.. ok.. nm..) continuing..
What are you concerned about? That the doc would refuse to see you for being trans? Unless you're in a small (spelled "close-minded") town or somewhere where there is little to no transgendered people, I would think it wouldn't matter what you appear in. As to what to say, just wait for (if any) the questions and answer as honestly as you can. If the doc is skeptical, remind him/her that there are thousands of trans men and women on the planet, made that way for a variety of medical reasons (DES Sons, for example) and should be treated no differently than any other group. Really, just be yourself, be honest, speak from your heart, really, it should not matter, the doc (again, unless you're in a small town) may have seen a trans kid or two, or knows of other practices with them.
As for ... Cosplay (dramatic fanfare music playing in the distance).. I have just begun to get into this, with watching SyFy channel's Heroes Of Cosplay show (was on last night). I LOVE Lauren, Holly, Inka and Ya-Ya!! (swoon). If you are on HRT, you may very well change where you are -not- read as male anymore, so the looking like a guy playing a girl probably wont happen with hormones, and the younger you are, the better (and faster) the results. everyone is different tho, but still, you likely will have good luck there!
Can you, maybe, promise, when you do a character, you'll post it for us? please??

I wish you luck, just be yourself, speak from your heart.
Hugs! Paula