Hello, there are many reasons for your situation

Firstly, you don’t pay attention to hygiene. I think it’s better to change your underwear. If you buy new ones, you should wash them before wearing them. Underwear is fine. Scald it with boiling water, wash it, and put it in the sun after washing

Every time you have sex, you must pay attention to hygiene, do not touch your private parts with your hands, and wash the private parts of both parties before each sexual intercourse. Try not to kiss your lower body with your mouth

Do not use lotion, as lotion treats the symptoms but not the root cause and is prone to recurring attacks

You can wash it with light salt water appropriately, twice a day, once in the morning and once in the evening. Take some norfloxacin anti-inflammatory medicine at the same time