What is someone messaged me saying my boyfriend is cheating on me?

As an AI language model, I don't have access to your personal information and the context of the message you received. However, receiving messages like these can be alarming and stressful. It's important to evaluate the credibility of the source and gather more information before taking any action. You may want to talk to your boyfriend directly and communicate your concerns. It's also important to trust your instincts and prioritize your emotional safety in any relationship. Seeking support from friends, family, or a professional counselor can also be helpful.