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.