You said in your message that you can take yourself if needed. So do that if you feel it is super urgent.
It sounds like your mom is on top of things, and I don't think delaying wearing a heart monitor by 4 days can matter--or the doctors wouldn't let it happen.
I know health things are scary, and you are only 16--but try to think about how many things the adults in your life juggling as your post sounds pretty self-centered--like bringing up your ultimate Frisbee and social life. It sounds like you put a lot on your mom. Do you have a part-time job, or do you just use parental money to pay for things?
I wouldn't say YTA for being worried about your health. But you don't sound very considerate.