Would the hospital tell my mum?

The medical staff will not discuss your health problems in front of your mother if you ask them not to. They are also bound not to reveal your health information to anyone that does not have a medical need to know. As you know your, mum may inadvertently become aware if she is present during an exam. It would be better if she were not present. The very fact that you are visiting a cardiac ward and talking to surgeons would definitely be a tip off. Especially if surgery is required. It is better if you tell her as soon as you can. Then she will not have to learn it any other way.