She told EVERYONE!!!

I KNOW YOU'RE PISSED OFF and you have every right to be.

First of all: file an official complaint about her to her supervisor. This is directly a violation of her stature as HR and as a medical info shared.

Secondly:  WORK IT! Use it to your advantage. Now that everyone knows, and you can't unring the bell, use it as motivation!  Honest to god, and I can only say this because I was a former fatty, trust me, people were talking about you behind your back BEFORE you had surgery! What they will be in shock about now is how GOOD an example you're setting, the excellent food choices you're making (and they'll be easy for quite some time) and how little you're eating. And watch with glee how the pounds roll off!

Blow this off. You can do this! Strut into work PROUD and with an attitude. I don't know what size you were prior to 25lbs ago but hey, isn't that amazing??? You'll kill off another 25 before you know it. It equals a small child!! There will be a spring in your step and a sparkle in your eye because ONCE AND FOR ALL, you're going to conquer this beast.

She may have done you an unintentional favor. ;-)

That was a horrible thing for her to do.  When people mentioned it, I'd be tempted to say, "Huh. I'm surprised you know about it. I only told [supervisor] and asked her not to tell anybody.  I guess she didn't keep it to herself.  Anyway, thanks for asking, but I don't really want to discuss it."  That will close down the topic for conversation and out her for blabbing.  It's kind of passive aggressive, though, so perhaps the direct approach would be better. 

Since she spread a true detail about your protected health information, she is in violation of HIPAA.  You need to report it to your human resources. It may be an issue of your supervisor not being aware that she is breaking a law and that there could be more serious repercutions from her action (lawsuit on company, herself, her immediate dismissal, etc.)