My mom won't let me ask questions. She MAKES me rephrase all my questions into statements. This prohibits my ability to communicate properly. She calls her rejection for questions a boundary, but I don't feel that it's healthy. She says she feels "overwhelmed" when asked a simple question, which is strange to me or abnormal at least. It doesn't seem like she's trying to work on her problem. Instead, it seems like she's just shutting out all questions and removing my ability to communicate how I prefer.

Is her boundary a real boundary, or is that just a shield? If it is a boundary, is it healthy?

It is important to respect your mom's boundaries, but it is also important for you to be able to effectively communicate and ask questions. It may be helpful to have a calm and open conversation with your mom about how her refusal to answer questions is impacting you and find a compromise that works for both of you. It is possible that her behavior is a defense mechanism or a way of avoiding potentially uncomfortable conversations. Ultimately, it is important to seek a healthy balance in communication within your relationship.