Your seemingly simple question has a lot going on.
first, do you qualify to be claimed as a dependent? There are two sets of rules under which you might qualify to be claimed as a dependent. One set of rules requires that you be disabled and live in your parents home more than half the year. “Disabled” for income tax purposes means that you are unable to perform substantial gainful work, and so, it does not mean the same as the word “disabled” might mean for a medical diagnosis. For example, someone who is blind or in a wheelchair might qualify as disabled by medical definition but is not disabled for tax purposes if they can perform a job of some kind.
The second set of rules under which you could qualify to be a dependent only requires that your parents pay more than half your financial support, and that you have less than $4300 of taxable income. You don’t have to be disabled and you don’t have to live with your parents to meet this test.
If you meet either test to be claimed as a dependent, you must answer “yes, I can be claimed as a dependent by another taxpayer“ on your tax return. For 2020 and 2021, this will disqualify you from receiving stimulus payments or the recovery rebate in your own name. If you answered “no“ so that you could claim the rebates, this might be considered tax fraud, and we don’t know how aggressively the IRS will pursue this. The IRS has three years to audit tax returns, and six years in the case of fraud.
Now we have to turn to the definition of “head of household.“ Your mother could only claim you as head of household if she also claimed you as a dependent, unless you were under age 19 and subject of a custody agreement between parents who are divorced or living separately. Using tax software, it would be impossible for your mother to e-file if she used you as a qualifying person for head of household but did not claim you as a dependent. If she filed by mail, the IRS computers should have caught the mistake and should not have processed a refund without requesting a correction from her.
Ultimately, the situation is this. If she claimed you as a dependent and a qualifying person for head of household, you must check the box that says “yes I can be claimed as a dependent”. If you answer “no”, you will be blocked from e-filing and you will have to mail your return, and the IRS will investigate both you and your mother after the busy season is over.
On the other hand, if you don’t meet the qualifications to be claimed as a dependent, you are perfectly entitled to answer “no that you can’t be claimed“, in which case you may be eligible for the recovery rebate and the stimulus payments. However, your mother may need to file amended tax returns in that situation to remove you as a dependent.
