Hi Meggie,
This is a difficult one and something that comes up time and time again on here. I too was unsure about telling my employers as I was relatively new to my job when I got my op date.
Thankfully my bosses were supportive and I'm glad now that I was honest as the rapid weight loss is impossible to hide.
Having said all that, I totally respect your right to privacy and if you dont want to tell people, then you dont have to. End of.
I would be inclined to say bowel surgery as that will stop the questions. No one wants to talk about bowels. And if they do ask you, you can say you have digestion problems and thats why you find it difficult to lose weight.
Its too risky IMO saying it is your gallbladder, as you may end up needing that out in the future as a result of the rapid weight loss.
As for the Ulcer, diabetes, heart problems..... I hope you dont mind me saying this hun, but be careful what you wish for.
I hope you find the right solution for you hun. All the best xx