You're farting but it's in your vagina sorta?

Fistula?: Sometimes this is related to a connection between the vagina and rectum. This would be typically following obstetrical trauma (episiotomy or tear). More commonly it's a mechanical issue related to obesity or loss of sphincter tone that causes the gas to pass either more freely or resonate near the vagina. Often the former is associated with walking. A thorough pelvic exam by a urogyn doc.