We all worked together and I had to have daily contact with her as she was involved with the admin of the contracts I let. She was always very sullen with me, to the extent that I said to my DH that I found her behaviour inexplicable and odd. He mentioned her quite a lot but I don't think he realised he was doing so - I think he couldn't help himself. Looks wise she was/is gorgeous so I put it down to that. But a gut instinct made me say to him (ironically), before I went away on a long business trip, 'try and avoid xx won't you as I'm pretty sure she has a thing about you which is why she is so off with me'. By that stage they were in the throes of a full-on affair..
After I found out and he officially 'ended it' - a lie I found out some time later - she was extremely overt towards him in the office and must have spent hours getting ready for work. I witnessed all this as I had returned from sick leave (I had been decimated physically and emotionally by the discovery of the affair) and had to endure her wearing skimpier and skimpier dresses and coming in and out of where we both worked up to 30 times a day. She was openly contemptuous of me, sent me emails saying how awful it must be to be ugly and old (yes, I did send them on to HR), and even surrounded herself with a small gaggle (coven) of women who made it quite clear to me that she would 'win' in the end and that my DH was with me only under duress/because of the children. HR/the bosses were hopeless and I finally had to engineer a job move myself as it was killing me to be in the same place as her and having to cope with her behaviour. I can't begin to explain how dreadful it was. It was bullying of the highest order.
This is a big jump but if your DH isn't in a physical relationship with her, I think it may be an EA. You need to have all your antennae on full alert - he is allowing another woman into his life and your relationship. She is entering the space you should inhabit. I hope I am wrong, I really do. x