Loss of Metabolism?

I use to be able to eat whatever I wanted and I wouldnt gain an ounce. I am 16,5'7,129-130. Now since I have been eating corrrectly without all of the junk food, I have lost the gut I use to have. But until about 6 months ago when I started boxing I could eat whatever and it wouldnt affect me. Now since I have been training and eating correctly,lately I have been throwing in a honeybun or some sort of little debbie cake and within a couple of hours, just from that little piece of junk food, my gut just falls right back into place. Then after a day or two, without eating any junk and still exercising it will once again go away. Is the reason it comes back so fast from something so small due to the fact of me losing my metabolism? Or because my body hadnt had any sort of junk food for so long it didnt know how to react to it or just what could it be? Any ideas would be helpful.

Thanks.

Most of it is probably in your mind (as far as eating one debbie cake, etc...) But being only 16 your metabolism is definitely going to change up, being you're probably not quite done growing in height and definitely not in natural weight. If you're training good don't worry about it so much right now. You're 16. Enjoy some debbie cakes if you want!!!