I know that this isn't what you want to hear, but telling your parents might be the only option when it comes to getting help for this, because as you've already said, they would have to deal with the medical costs. However, are there sexual health clinics that you can go to for free for advice and such? I live in the UK, so it might be a bit different, but over here, we have free sexual health clinics aimed at teens where we can go to get tested for STI's and ask questions, as well as have access contraception and things like that. But if there are no such things where you are, then you could tell your parents that you aren't having periods; they don't have to know why, and go to the doctor because of that. The doctor is not legally allowed to say anything to your parents unless you want them to, I think, because they have to practice patient-doctor confidentiality, meaning that your parents won't have to know the cause of your problem; you could make something up about a hormone imbalance or similar cause. I hope I helped
