First of all take a deep breath - What you did was wrong regardless of the circumstances. It is never appropriate to strike a patient even lightly. However just because an investigation is being done doesn't mean that the sate will find on the side of abuse. All healthcare workers and professionals are mandated reporters and as such are required by law to report "Any reasonably suspected abuse to the regulating agency". Where I work if something is observed or reported the person involved is immediately suspended while an internal investigation is conducted. If they end up reporting it to the State that person is usually terminated. However if the incident is not found to be abuse the record of the investigation is filed and kept in case someone does report it and the State comes to do their own investigation.
If you are terminated remember you have a legal right to review and ask for a copy of your personelle file. They cannot deny you this. You should also ask them to show you the documentation of their investigation into this incident, although they don't have to provide you with a copy. If you really feel that you are being unjustly treated you can hire an attorney but that may open a S-storm of trouble.
Whereever you work in the future I recomend that you take certification in Non-Violent Crisis Intervention. My company requires all new hires be certified before they have patient contact and that they recertify every two years. This training helps you to know what to do to prevent those reflexive responses.
Good luck to you
Hppy