At first glance I got nothin'. If I over analyze I don't end up thinking there's another OM, necessarily.
Maybe it's just me but I don't get asked out every day by some random dude so I'm assuming unless your W is on an active dating site or wearing a sign that says, "hey, i'm single, date me" she's not prancing around with another OM (especially if she has the kids a lot).
Her text, "I'm not going to do dinner" could easily just be, "my tummy is upset and I don't want to deal with diarrhea while in a restaurant." Not to be tactless but there are a 1,000 things one could interpret with that. She didn't say, "sorry, I can't do dinner, I made other plans" she just said she's not doing dinner. Maybe this is HER way of being vague so you'll do exactly what you're doing... wondering.
Show her it's cool. Water off a duck's back, my friend.