Chasing a girl, she was fine, but she suddenly stopped returning my messages. What should I do?

It depends on what kind of girl she is. It may be a little trick that she is interested in you, but she doesn't want you to get it too easily. Isn't that what love is like? Cunning little tricks are mixed with all kinds of natural contradictions. I don't think you need to know something that makes you understand so quickly and clearly. Just feel the obscurity of the moment yourself and it will grow naturally. As for wanting to know if she is boring to you and ignores you, you can politely ask her to give you a clear answer, but what she said may be duplicity.