What does your girlfriend mean by "I'm fine"?
For example, if a girl says she is fine, there are several possibilities for her real meaning: the first possibility is that I am really fine now. The second possibility is that I have something to do now, but I don't want to talk about it. I want to be alone. And the third possibility is that I do have something, and she very much hopes that you can guess what is in her heart. Although he has kept silent and hid his unhappiness deeply, he hopes you'd better coax her to be happy in time. In the first of these three possibilities, girls are the least likely to be really fine. The third possibility is universal.
Of course, it may not help you much if you just guess out of thin air. You can make some confidants and better friends of the opposite sex. Ask them for their experience and methods in this field, and let them help you analyze and provide you with more accurate reference. Let you really guess what girls think. But the most important thing in interpersonal communication must be remembered: keep your sincerity and kindness. Regardless of boys and girls, everyone likes sincere people, and girls prefer the sincerity of boys. Including their thoughtfulness and nuance.