It could be thrush/yeast infection.
Try an antifungal cream and stay away from sweets, refined carbs and alcohol for a while.
Virgin Organic Coconut oil, topical and ingested might also help.
Are there other related symptoms, itching, burning, etc.? Also I will add that keeping the area clean and dry at all times will help. This is hard to do in this location and bacteria grows well there. Switch to cotton panties, these wick moisture.
There is also an anti-fungal powder which will help both functions.
Without other symptoms it could be allergies. Try also taking 1 allergy relief tablet at bed time for several nights. If you are wearing pads you may be allergic to these try an alternative. If you see improvement then your doctor is correct.