Now that you are 18 years old. Go to the bank with all the identification you have and remove everyone from the savings account, or simply withdraw all but $20 and open up a new account at another bank.
If the account was never in your name, if your parents were divorced, then it seems unlikely that your mom would have access to the account, so she could be messing with you.
If the savings account is in your name, then you should have access to it. Since your mom's name does not appear on the account she should not be able to access it. Just go to the bank and withdraw the money.
You may not have all the facts. The assets could be in a custodial account or even a trust.
Go to the bank yourself and get the facts.