Though a youth pastor might be helpful in the spiritual sense, you need to see a doctor to help you with the physical. Perhaps you can have your mother go with you to the appointment, but ask the nurse ahead of time if you can talk privately with the doctor. If your mother hears and wonders why, just tell her you get embarrassed with her in the same room…after all you’re older now. It is common for parents of teens to wait in the lobby.
If you're afraid your mom would be upset, then after the appointment is made, secretly call the office yourself to notify them ahead of time that you need some alone time with the doctor because it’s hard for you to talk openly with her right there. They should be accommodating without blabbing to Mom. When I was a teen, the doctor always talked to me privately. Only the really young need a parent in the same room. The main reason doctors want parents at the office, is for insurance and payment information or to approve any medical procedure. But consultation and exam should be parent free. Most doctors know that many teens won't be open with Mom or Dad right there. Hope that helps.
BTW: Your weight sounds ideal, especially if you are lean - muscle is heavier than fat, so you are thin. Try to eat several meals throughout the day to maintain your bstrength and health. It would suck to damage yourself long term.