There are multiple questions here so I'll tackle them one by one.
Your initial question: Can a gummy bear give me diabetes?
One giant gummy bear, alone, is very unlikely to give you diabetes. I don't want to say indefinitely, because nothing in health is a definite. However, the likely hood is so close to 0% I would consider it insignificant.
Next, you stated that immediately following the ingestion of a cute gummy bear, you felt tired and the next morning your breathe smelled sweet.
The reason for both of these symptoms can very well be due to your sugar levels becoming drastically high. In the medical field, we call this DKA. (Diabetic ketoacidosis). Essentially, your body recognizes a sharp increase in blood glucose levels and is trying everything in the books to expel the glucose. Your body does this by releasing ketones from your respiratory system. DKA is most notably recognized by sweet smelling breathe. Almost like juicy fruit gum.
DKA can actually be a very serious emergency which, if unchecked, can lead to a diabetic coma. However, for individuals without diabetes, and a properly working pancreas, your sugar levels should be back to normal by now.
I believe your incident with the cough is completely unrelated.
To recap. No I do not think you have developed diabetes due to a single gummy bear. However, it never hurts to be checked for the disease. You could very well already have diabetes, and the gummy bear threw you into DKA. Because I cannot see you, nor properly perform any tests. To cover myself for any legal issues I will say that 1. I am not a doctor and 2. Go get it checked.
In regards to telling your parents. Just say you had some candy and you felt really weird afterwards and you're nervous that you might have diabetes. Any sensible parent will at least set you up with a doctors appointment.