Laur, are there employment privacy laws that would prevent your boss or the human resources department from disclosing ANY of your medical issues or medical history to anyone in the company? Could you find out by researching your company website or government website related to sick leave and disclosure? It might make you feel safer and comfortable when the time comes to talk. When I told my boss I would need sick leave for several months for major surgery I did not specify what kind of surgery, or what it involved. Too gory. My medical history isn't their business and they did not ask for details. I filled in forms for leave, with a letter from my doctor who wrote "surgery" with no other details.
Remember that some people can't resist talking about someone else's medical problems. If you do not want anyone in your workplace to know the details of your UC, or explain what a j pouch is, simply don't tell anyone else. No one can force you to disclose your personal medical details, even if they choose to share theirs. No quid pro quo. I've found that most people have no clue what a j pouch is, and the explanation and details are just too complicated. I've known people in the medical and health care field who do not understand what a j pouch is! If you do not want co-workers talking about you, or trying to figure out what a j pouch is, maybe you could simply tell them it's abdominal surgery to give you back your health? Ask for their support and say you hope they understand your need for privacy. Don't feel you have to trade personal information to get sick leave.
Try not to worry about how to tell your boss or human resources. Do not worry about how your absence will affect their workload. It is their job to figure out how to spread the work while you're away. They are paid to do that. Your health is the most important thing to preserve and protect. If they needed surgery, you would shoulder the load for awhile, wouldn't you? Anyone can find themselves in your shoes. Write down key talking points and stick to them. Decide how much to tell coworkers, and stick to it. Now might be a good time to quietly research if your (company's?) insurance plan covers medication and the ostomy supplies when you have the temporary stoma, so you can plan and budget. When you're in some measure of control, you won't feel socially awkward. You will be okay.