The texts do make things look bad, however, the fact that your mother replied "let me think" is a good sign. She may just be having a emotional affair at this point, but she is not totally pulled in. I wish I could reasure you but it is hard to tell how deep this relationship is with so little information.

You might consider having a conversation with your mom about this. Tell her what you found and let her explain.

Whatever you do, I wish you the best and hope your fears are unfounded.