it is not good or bad, but it is normal. i wouldnt worry about it it happened to me all the time.
You should NEVER force air UP through your vagina, especially during pregnancy. This means no douching or other activity that might push air up there. I am not sure if air can come OUT of your vagina, but I would NOT think it could be harmful if it was coming out naturally.
Hmmm thats a good question. I am 6w 4d and I have little bubbles of air coming out of my vagina every now and then. I have no clue if thats normal or not!