Hello this needs detailed consultation. It seems to be fungal infection. Also you need to maintain proper hygiene. As you said this happens during menstrual period so i need detailed description as to for how many days itching persists and other days is itching totally absent and you are totally comfortable?
Next Steps
Wash your inner wear in hot water and then dry them properly under sunlight. Also iron them before wearing.
What about your sexual history? Are you married?
Health Tips
Consult me online for detailed history and then medications.
For now once you go to washroom for urination everytime kindly wash the area and wipe the area dry before wearing your inner wear. No wearing wet underwears.
Apply neem oil in the area. twice once while going to bed and once in the morning