Why did she show the short message I sent her to my roommate in our dormitory?

Well, first of all, I don't know what her personality is. I can only analyze it according to your literal question. We can consider how much you are in love with her first, and then combine her personality analysis. If she likes you very much and is a cheerful and lively girl, in fact, such a move is not worth making a fuss about. If she knows your roommate well and knows you before you, it's more normal. Men can't be too possessive, which will make each other feel insecure. If the message you send her is to express your love, then when she shows it to other men, she can basically be sure that she is not having an affair with others. If the content of the message is other content, it is suggested that she consider it comprehensively. Of course, it does not rule out that she has an affair with other men, so be vigilant and pay more love than usual. This stage can be easily recovered without anxiety.