3 weeks after baby's birth and having air trapped in vagina feeling?

Queff. It is common to have a wide open vagina and spacious internal cannal after childbirth. This can lead to a negative internal pressure of the vaginal canal that draws outside air inwards. That gas can the be expelled when there is an increase in the intra-abdominal pressure leading to an embarrassing flatulent sound. This is a "queff" in slang terms. Vaginoplasty surgery can reduce this problem.
Vaginal air. If this was your first birth, your vagina was stretched quite a bit, air trapping with noise upon release can be normal , during or after sex.