Boyfriend suddenly received a text message from a girl?

I understand your worries and doubts. First of all, your boyfriend has explained to you frankly that that girl is his predecessor and has always been cold to her, which is a good sign. However, it is normal for you to be unfamiliar with the contents of those short messages. If you are uneasy about this information, you can ask your boyfriend directly and let him explain further. At the same time, you can also consider establishing more frank and transparent communication with your boyfriend to make the trust between you more stable. Finally, if you feel troubled, you can consider seeking professional help, such as talking to a psychologist.