More information is needed. in general, a person who represents themselves as a U. S. Citizen is allowed to vote. An alien who "falsely represents" themselves to be a U. S. Citizen is deportable. The issue is whether you represented yourself as a U. S. Citizen.
County Clerks have made mistakes. Some DMV Agents insist that all drivers fill out the voter form, even when the box will be marked no to citizenship.
If the box was marked that you were a U. S. Citizen on the voter registration form before you signed, then you likely affirmed that you were a Citizen. However, sometimes there are exceptions.
I strongly recommend an appointment with a candid, competent, ethical and experienced immigration attorney before you file any forms with the USCIS. Do so soon, since forms may be misplaced by county clerks, among other administrative challenges.
This is general information, not legal advice, and does not create an attorney client relationship.
You should definitely meet with an immigration attorney to help you. Maybe a FOIA is in order before filing. Also, did you register to vote in the federal elections or in state elections? These are two different issues.
This information is intended as general information only. This communication does not establish an attorney-client relationship between me and the asker.