I’d begin with asking your girlfriend to have a discussion with her gynecologist about the onset of her shift and taking the birth control pills. She’d want to rule out any medical causes before jumping into unraveling the psychological.
If it turns out the pills or a medical condition aren’t the issue, and it remains clear that your behavior with her isn’t the trigger (meaning you are not saying or doing something specifically that has been unacceptable to her), then I’d suggest she work with a therapist proficient in working with people with sexual inhibition.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral