Thanks for your questions. One thing that's important to understand is that you don't necessarily have to major in biology or even chemistry to be accepted into medical school.
Here are two links that will help break this down further:
Association of American Medical Colleges - Premedical Tips and Advice
In essence, it's more important that you A) Do well in college overall B) Take the required pre-req courses for medical school (many of which can be fulfilled by completing electives in college.
Also, you are taking AP Biology which is a college-level course which means you already have an idea of what science courses will be like in college. A good academic advisor in college will help you select the right courses so you can find some balance meaning you can manage somewhat "easier" classes with courses you know are going to be more difficult like Organic Chemistry for example.
I hope this helps and please continue to post your questions on this forum.