Do I really have a big mouth?
That would depend if you are able to explain why you think you have a big mouth?
I guess. If I don't, could you please tell me yourself?
Since you are aware of your big mouth, why do you keep on doing it? You should not keep saying things without thinking, it can cause a lot of regret, anger, and judgment from other people.
I only keep my mouth open when I know I know the answer.
It sounds like you might just know the answer to many things, because I don't think you have a big mouth. It might just be that you are being oversensitive. I think you are just really smart and have a lot of knowledge, which makes you want to talk.