You still feel that way even though if I didn't happen to see who she was texting I would still not know they are texting?If she was texting sitting next to you, then she wanted you to see. (And if she didn't want you to know, she could easily have made up some story about who it was). She is giving you a warning, in a rather indirect, manipulative way.
Don't dwell on the "other guy" situation. Don't talk about it or demand answers. Focus on making any changes in your own behaviour that you want to make, (and on finding another job). Change yourself in whatever way you think is right. That's my advice.