Since word is out, not much you can do about it. Don't provide any more details! Most people will not be content with knowing about the surgery. They are going to want details!! How much did it cost, how did it go, how much can you eat, what are you eating, and the big on: HOW MUCH DID YOU LOSE??? SAY NOTHING!! that will kill them on the inside, trust me lol!
Your boss took away your choice to keep this a secret, but you are in control from this point on. You got the time off that you needed, so forget about her. But from now on her and all the rest can only watch you get skinny but not be privilege to any more information.
I am so sorry you went through that, but you have control of your life again!! best of luck dear.