I think she's just tired. Get a good babysitter and take her away for an over-nighter where she doesn't have to do anything but enjoy you and her surroundings, a nice dinner and a relaxing hot tub (get a room that has one).
OP: Don't let people talk you into thinking that she's having some kind of affair. There is no proof that she is and putting shit like that in your head does nothing to help you and her get back the emotional connection.
She works full time and she looks after an 18 month old. She probably doesn't feel attractive or sexy and a woman's mojo starts working when her brain is engaged. She's checking out because she's lost her own identity as a viable, attractive woman (very likely).
I'm married and I've had children and I can tell you without a doubt that you need to start treating her like you and she are gf/bf and not the mother of my child.
Stop texting her and pick up the bloody phone and flirt with her. Texting is burdensome to someone who is busy and can't be bothered reading words on a screen without actions to back them up.