My SO use to be just a sex kitten, now it's not even on her "to do list" with me.  Instead she's turned into just a terrible D-tease.   She'll get home from work and watch tv, read the paper, drink a few beers and fall asleep on the couch. How does a woman just shut it off ?  
My only explaination is I suspect she has someone else she's doing it with.  Anyone relate to this ?

In many ways, yes.

First, don't just assume she's doing it with someone else. She may simply be bored or unhappy with your sex life. She may feel like she's being taken for granted and wants a little more attention and romance. She may be disenchanted with the relationship. She may be depressed.

In general, women in a relationship are not enamored of sex by itself. It requires a sense of being wanted, being worth chasing, being worth going the extra mile for.

Any way you look at it, you need to open up some communication with her. It's not easy or comfortable, but if the relationship is at risk, it's worth it. Talk to her, ask her what is going on, tell her your concerns.

If it helps: in my experience, a woman who is having an affair is *more* likely to be sexual with her spouse, whether that is from guilt or feeling sexier or something else entirely.

I mistress says "Harder, harder".
A prostitute says "Faster, faster".
A wife says "Beige. I think I'll paint the ceiling beige".

Appreciate your insights and thoughts. I suspect you're right on the mark with this.  I think it takes very few things (demands on their time, children issues, job etc.) thrown into the mix to put them out of whack. Being the rather unpredictable creatures such as they are, basic "guy" daily planning items such as  prioritization and time management arn't always on their agenda.  You're correct that being understood and chased are what they want.
However if she went from pulling 8, 9, and 10's in the sack to 3's (like last night) it raises a red flag.  Cheers!