Seeing the short message from my girlfriend, I feel that she is cheating, but I'm not 100% sure what I should do, whether to ask her or pretend not to know. I'm confused.

What is the content of her short message? You should ask, maybe you are too sensitive, she didn't cheat. You should summon up the courage to tell her that if you don't ask clearly, there is a thorn in your heart, which will affect your feelings, or she is cheating. Are you willing to be an idiot without asking her? Other things can be vague, and feelings must be clear! I wish you happiness. Can you accept my answer as the best one?