It's hard when no one believes you. I have pains in my knees, and I told my parents about it when I was your age. For the longest time they believed I was imagining it, or that it was growing pains. They thought that I was over reacting too. Until after a few months, I finally went to a few doctors where it was discovered I have tendinitis and floating kneecap. Even though my parents didn't believe me, I kept complaining, and asking to see someone. They told me it was in my head, which made me feel bad that they didn't trust me enough, but I kept pushing it. I think that's something you have to do as well. Tell them it hurts, or when you go to the doctor next tell them. If it's really painful, make up an excuse to go to the doctor. Say you don't feel well. But don't let something affecting your health get out of hand. You don't know what it is or if it could become a long term issue if not taken care of soon.

As for your memory, this is something else that shouldn't be ignored. You need to tell someone, preferably someone who can help. This is also something you should be discussing with your parents or a doctor. Someone who can get you the help you need. For some reason your brain is repressing memories. This could mean something traumatic happened your body doesn't want to remember, or there's a problem with your brain. Either way, it needs to be taken care of. Something that you don't want to ignore and want to get checked out right away.
Again this could be hard with no one believing you. And look at it from their point of view, it's hard to believe, or it's something they don't want to believe because they don't want to deal with it. But the fact is it's happening, so you need to find some excuse to talk to someone.

There is the option of getting to the doctor by faking sick, or waiting until your next check up, or just plain old asking. Maybe say you have something you'd like to talk to your doctor about you're not comfortable talking with your parents about. I'm not quite sure what kind of professional can help with the memory loss sort of thing, whether it's a psychologist or something else, but your doctor will be able to refer you to someone.
Another option is a school counselor. No, they are not trained to help with your memory problem, but they are trained in dealing with parents. Say what's going on, and let them know how your parents are not believing you. They can help you work something out to get your parents to get you the help you need.

If you have any more questions on how to possibly approach someone about this, feel free to ask.

That's pretty horrible thing for your friends to do. Have you asked them to cut it out?
My friends did that too until they knew how bad my knees were, so they stopped, and even stood up for me if someone else did it to me. Maybe telling them might not be such a bad thing, even if it risks them not believing you.