while the process of giving birth,There dialatation of birth canal.mother has to bear down.due to the force compleate pelvic floor
including anus gets dialated & loose.That's why air can enter loose bowels bringing feeling of a bubble in anus which escapes out.
To get rid of it one should do pelvic exercises after & for ever.i.e.-tightly close &pull the anus up while inhailing .
-release the pressure to leave the anus loose while exhailing.
This gives tone back to both anus & vagina .