Hello and thankyou for approaching DoctorSpring with your query.
I have gone through you case and understand your concern.
During sex, due to sexual stimulation and excitement there are increased secretions in the female genital tract. The discharge could probably just be normal .

However since you say that you are having itching, it may be that your girlfriend had a candida (fungal) infection. It occurs frequently in female and can occur even in healthy individuals. It is NOT a sexually transmitted disease.

In case she did have candidiasis, you are probably experiencing are reaction as a result of exposure to the organism or the antigens in the vagina. Male partners develop postcoital itching and burning with redness or rash on the penis. This is nothing to worry. You need to have a shower immediately after sex to avoid this.
For now you can apply an OTC cream Resolve plus twice a day.

If your symptoms are repeated after every sexual encounter, you will have to very sensitively broach the topic with your girlfriend. She will have to get tested and take a course of antibiotic for the same, which will clear her symptoms.

I hope this has helped. Feel free to follow up.
Thankyou

Yes it is available OTC. Another option is Lotrimin cream. Your girlfriend might need vaginal tablets or oral medications, which is prescription only. It is better if you get a standard STD test done.

Thank you