I accidentally saw a man on my girlfriend's mobile phone sending her an ambiguous message. I said a few words to her, and she got angry and ignored me. She said I didn't believe it.

Men and women may like to have an affair. Because they like the feeling of being spoiled. Even if that person is not his boyfriend or girlfriend.

Or, really, there's just one guy who likes him better. Just harassing him. But I suggest you observe more and find a chance to see the information your girlfriend gave him back, so that you can know what the situation is!