I have a bumper sticker thumbtacked to the wall in my office:
"No one can make you feel inferior without your consent," by Eleanor Roosevelt.
The point: sure, what your friend said was horribly unthoughtful. But who's the one who should be ashamed here? You or her??? You have no reason to feel embarrassed or humiliated-- YOU'RE doing great, right? It's your friend who seems to have the issues; if anything, you should feel embarrassed for HER.
Be proud of yourself and your accomplishments. So what if anyone else in the dressing room heard her comment about your weight-- what difference is it to you WHAT they think??? Most of us go through life far too worried about what other people think of us, when in reality, all those other people are just thinking about THEMSELVES anyway and couldn't care less about some stranger they saw for 30 seconds in a dressing room.
I'm not trying to make excuses for your friend, because clearly she's an idiot.
But really, it's not worth getting so upset over-- if you're happy and confident with yourself, there's no reason to allow her comments to take anything away from that.