What is wrong with my hair/scalp?

I'm 15 now. My hair used to be really nice, and I had no problems with it, but for a couple of months now, it has gone very downhill. My hair used to be very thick, and it still is, but I feel like it's starting to get thinner on the top. Also I'm losing a lot more hair than normal, I feel like. Also my scalp is always like flaky, and has gunk on it. I used to be able to go for a week without a wash and without it getting greasy, but now it get's greasy and has flakes after just 1 day. I've started taking multivitamins in hopes that it will help. I'm going to the doctor soon, but I'm still scared. Should I go to a dermatologist? Should I start using head and shoulders shampoo for my flaky and oily scalp? Could it be because I'm not eating right? What even is wrong with me? Please help I'm scared that I'm going to be completely bald or that my hair going to be thin. So much hair is falling out.