Hello,
This can be due to vaginal flatulence which is the expulsion of trapped air from the vagina. A common method for reducing instances of vaginal flatulence is to strengthen the vaginal walls and pelvic floor by performing Kegel exercises. If it persists then a rectovaginal fistula and genital prolapsed should be ruled out.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.