I saw your post to Sandi on the the other thread. IMO your W is definitely lying yo you and I would be completely shocked if she wasn't already dating OM. If not she most likely would have at least waited until after the holidays.
Shake off what you told her and keep moving forward. I have yet to see a story where the W fast tracks to D and then changes her mind a month or so later. It is usually years down the road.
Just remember all these proclamations you are making are based on how you feel right now. Your feeling can and most likely will change in the future.