Without knowing the specifics, I'm guessing you were a dependent on your mom's 2018 tax return, her 2019 tax return, and possibly will be on her 2020 tax return. Unfortunately, only children 16 years old and younger in 2020 were considered qualified dependents in calculating the amount paid to parents for the first and second stimulus checks (which are considered 2020 tax refund advances).
If you turned 18 in 2020, filing your own tax return, and are no longer a dependent on your mom's 2020 return, you MAY qualify for the Recovery Rebate Credit (which is an adjustment if you did not receive, or received the wrong amount of the two stimulus payments) on your 2020 tax return. However, I'm NOT positive about this. I believe if you use tax software it should prompt you to answer questions which determine your eligibility for this Credit.
HOWEVER, If you are still your mom's dependent, the proposed third stimulus payment (currently $1400) WOULD allow your mom to receive payment for you as an adult dependent in the calculation for her stimulus (at the same amount, currently $1400).
I'd STRONGLY advise you to check with your mom as to whether she is correctly planning to claim you as a dependent (e.g., you're a student and/or living with her and she is supporting you) BEFORE you file as a non-dependent.
If you can be more specific about your 2020 status (are you a student? do you live at home? what was your 2020 income?) that might help as well.
If I get more info I will post again.
