Sonja,
Tough love time. Put yourself first. You're 34 with 5 full abdominal surgeries and now unwell. Let your boss know you have to go to emergency or you will collapse at work. Your body is trying to tell you something: take care of me first or I will give up on you. Could you survive on permanent disability? Will your colleagues, who won't come in to take over for you, come to take care of you if you are ill, cook and clean for you?
Your health is more important than anything else, so get them to assign someone to share the key carrying responsibility with you. They got used to you falling into line so they don't need to get someone else to come in. Why should they rock the boat when you are willing to go in. Protect and cherish your health. You don't want to get so bad that you finally lose your health permanently. Your company might send you a bouquet of flowers / get well soon, Sonja, and they'll move on to the next key carrier. Unless this is your own company or you own majority stock and there is no one else, get yourself to the doctor. You know on airplanes they tell you to put the oxygen mask on yourself FIRST before you try to help another passenger? Please look after yourself so you can look after others when you open your smoothie bar.