Your story does not to me mean that it is an act. Can one not go from genuinely concerned to happy when faced with another person in front of them. She could have been genuinely concerned that you were struggling and then, when it turned out you were not, she was genuinely happy for you. I am not sating that there is not sometimes an act, but are you not over thinking her actions?

Sorry if this sounds mean in anyway, it is not meant as such. I am guilty as ever of a large amount of over thinking and wish I could reduce this but it is who I am and I think learning to accept it may better serve me.