It might not be due to oral sex but due to routine bacterial urinary infection acquired by intercourse that u do with ur partner.
Getba urine analysis n culture done nest time it burns.
However to be safe, Both of u should Get tested for
STD s... A dermatologist would help
And ensure good oral and private parts hygiene - all ladies must use vwash ( or similar brand) down there before and after sex. Also not to apply soap there ever .. either water or vwash
Both should Pee before and after sex.
Use condoms till u get cured .
U should wear loose fitting cotton undergarments n trousers / leggings always
All females must wipe stool in such a way that dirty water n hands dont reach the vaginal and urinary area. If u use jet to clean after passing stools-- use a hand jet with direction of water from front to back
Never use a fixed jet which pushes all dirty water from anal part to urinary area In front.
Take lot of curd and buttermilk in diet.
Drink 1 glass water every 2 hrs