Medicaid is based on household income of everyone living there. If you failed to report your GF income or anyone living with you, then that could make you ineligible.
If in fact you have not been to a doctor in 2 years, then prior to that what kind of treatment did you receive? For $21,000, would seem like surgery or a expensive procedure that was done, or multiple trips to the ER.
It could be that due to this being Medicaid and they are slow in processing claims, or did an audit and found these charges and questioned them, thus the letter. Or, it could be a possibility that someone else has used the Medicaid fraudulently and you were not aware of this.
You will need to make an appointment with your case worker to work this out.