Call the facility right now and do not wait. Someone passing out repeatedly and then going silent for two days is a situation that warrants an immediate welfare check, and you have every right to ask for one.
When you call, ask to speak with the unit team secretary first. Explain clearly and calmly that your girlfriend has been calling you daily, that she had been reporting episodes of passing out, and that you have not heard from her in two days. Ask them to check on her welfare. That specific combination of information, the medical concern she reported plus the sudden communication gap, is the kind of thing that staff should take seriously.
If the unit secretary cannot help or gives you a brush-off, ask to speak with the counselor or case manager assigned to her unit. Do not hang up until you have spoken to someone who can actually walk to her housing area and confirm she is okay. Keep your tone concerned and cooperative rather than panicked or demanding. Staff who feel they are being approached with genuine concern respond more helpfully than those who feel they are being pressured.
A few days of silence can happen for several reasons including medical housing, a disciplinary situation, or a transfer, but given that she was telling you she was passing out, the medical angle deserves immediate follow-up. If she is in the medical unit or infirmary, phone access may be restricted which could explain the silence without anything catastrophic having happened.
If the facility cannot or will not confirm her status after multiple calls, contact the state's department of corrections ombudsman or inmate advocate office. They have the authority to request welfare checks on specific inmates. Do not wait. Make the call today.