During arousal, the vagina lengthens and the uterus moves, creating more air space.
Extra lubricant can make a difference.
Ask your husband to use some before he enters you in those certain positions. For a while, you also may want to stay away from the particular positions during which this happens, to spare yourself the embarrassment. Remember, what's happening to you is completely normal. Click on her picture for her bio. OR Questions may also be posted on her Relationship Advice Board.