Welcome to HCM,
It seems that your girlfriend might be having Trichonomus vaginilis, a fungal infection
Now due to itching that part became inflamed due to infection.
Go for one antibiotic medicine course along with anti inflammatory medicines.
Go for antihistaminic like Cetrizine
Apply triple acting cream like micoderm-g,Multifungin-f.
Afterwards get examined by gynacologist and go for the treatment of fungal infection.
Ok and bye.