I think this could have been so easily solved.
"Are we okay"
"Not really, you broke a confidence and I'm upset with you about it"
"I'm sorry"
"Look, I am hurt, but I'm sorry too for putting you in a situation where you may have felt awkward, I didn't think it through"
"No, I'm really REALLY sorry"
"I accept your apology, do you want to come the Egg hunt?"
Everyone hugs. Peace is restored.
There's no malice involved from your friend OP I doubt there was any from you either. This bearing of a grudge though, that your friend failed in her assigned task, will win you more enemies than friends.
And though I am sure you are eating healthily and exercising, your drastic weightloss is not just down to sheer hard work, its down to sheer hard work and an operation. From what you've said, you're in denial about the extent of the affect of the operation, that YOU somehow feel you've cheated, hence the HUGE amount of offence taken at your friend messing up, you'd rather people assume you're some dieting wizard than them know you've had potentially life saving surgery. You haven't cheated and I suspect your friend, that you're now ignoring and cooling off from would be incredibly supportive of you if you'd let her. She already HAS been supportive hasn't she? I think you're being hard on her.
I do think that although you shouldn't have to advertise your surgery, making it such a secret only serves to make people more interested.
By the way, lady I know had surgery 3 years ago, has lost a mountain of weight, and has been open about it to anyone who's asked... she's not had one criticism only support, not had anyone gossiping about her, and I certainly am full of respect for her, because she works damn hard to make it work. Don't advertise if you don't want to, but be proud, and give your friend a hug.