My Girlfriend (22) Has Recurring Internal Burning/Itching After Sex – No Diagnosis Yet

My girlfriend (22F) has been experiencing a burning or itching sensation (she's not entirely sure) inside her intimate area since we started having sex. It doesn’t happen immediately but usually appears hours later, especially after movement. Over time, it started occurring even when we hadn’t had sex recently.

She has been examined by a gynecologist, but nothing was found. A treatment for a yeast infection was ineffective, and I have been tested for STDs – all clear.

There was a suspicion of a UTI, but regular antibiotics didn’t help. She was then prescribed a reserve antibiotic, which seemed to work… but now the symptoms have returned.

The only thing she has noticed is that drinking more water seems to help.

Does anyone have any idea what this could be?