Dear,
From whatever you have described, it seems your girlfriend might be developing symptoms of depression and may be even psychosis. However, nothing can be ruled out without formal investigation.
Next Steps
Please immediately see a psychiatrist or a psychologist. Even if your girlfriend is not willing, you or someone who knows her can see the mental health professional and describe the case in detail. They can guide you with appropriate management.
Health Tips
If possible, inform your girlfriend's family if they are not aware of her these thoughts. Try not leave her aloof.