You will have to scrutinise your policy very carefully and if you are not sure, telephone the insurance company.
Years ago, most fuuly comp policies were 'any car, any driver' type - the person who was named on the policy could drive any car with the owner's permission and anyone could drive the insured car, although both these scenarios were covered third party only.
Recently there has been a clamp down - mainly to prevent fraud and many companies will charge a high premium for 'any driver' cover, or exclude it entirely.
Your insurance may allow you to drive another person's car because they will not pay out for any damage you do to that car, and your own car is safely parked up somewhere else. They'd have to pay third party claims, but that would be the same if you were driving your own vehicle and had the same accident.
Without looking at the policies I would guess that no, your girlfriend is not covered to drive your car.