My heart goes out to you ... Perhaps you are right that your Mom is not the best person to help you get through this. Best friend who can be trusted to be strong for you?
The doctor should know in order to do her best to be senstive to your very real fears.
I allowed my doctor to do the exam a few months ago because it had been 7 years. Even though male, he was awesome. I've read a lot about "physical evidence" abuse and it is my understanding that even in children, it is very hard to tell there has been assault after even a very short time - heals very quickly, so it is unlikely that there is damage. However, some people want to be reassured that there isn't. I told my doctor that I wanted to know if there was, because it would be "proof" for me.
What is best for you? If you tell your doctor before-hand, in the end, it will be easier. There is no doubt that this is a very hard thing to have to go through, but it is a relief when it is over and you know that all is well. Whatever you do, do your best to say what you need and don't need. Feel your power!
I truly hope that this will be okay for you, and will be thinking about you.
if safe