Talk to her as a team:

Her behaviour of standing outside the door shows that she wants to have total control over what you both are doing. You can speak with your husband first and then speak to your mother-in-law together expressing your concerns and that you both deserve some space. You can discuss this with a family counsellor also on how you could effectively communicate with your mother-in-law. Her behaviour could be an outcome of an insecurity that her son would get emotionally detached from her and sees you as competition.