Hmm, depends on where your H works and whether the people who work with him love drama and shit-stirring, perhaps are quite young? If he works in a corporate-type environment then I'd say more likely to be OW.
Other question to ask is how this person would have got hold of your details to send you this message. Was it a text? I'm guessing not FB as you'd be able to see the originating account. Much easier for an OW to access your H's phone and note down your details than a random work colleague, if you see what I mean.
The only way to know would be ask if it was her, the OW, as you'd like to know some more details. But even then she could deny it was her so you're no better off.
Probably best to make peace with the fact that in the brand scheme of things, it doesn't matter who sent it. Are you worried that it is someone else/ a work colleague and feel embarrassed that people know?
can you do a test? thank the messenger and say I'll deal with that after I get back from Belgium. Or whatever. If a friend ever says to you 'how was your trip to Belgium?' you'll know. Am I too columbo?
or maybe reply "are you sure? how do you know, things have been so good between us lately" if OW, that'd cause trouble.
BUT if you know it's true, I wouldn't agonise over who sent it.
Because basic employee contact details (including 'who to contact in an emergency') are usually easy to find out in a workplace, so any number of colleagues might be able to get hold of them.
So yes, it might be the OW forcing a crisis (New Year does that to people). Or it might be that a colleague has noticed something going on and thinks you deserve to make decisions for your own future based on truth not lies (see oodles of past MNnthreads on 'should I tell?').
I would reply asking for more detail, and go along with the line of thing being so good between you right now. If this has come from someone else they will tell you, if it's come from an ow it would probably upset/cause problems for them.
I had the same thing and I think it was a friend of XH's who was disgusted at the way he was treating me. At the time, I thought it was a malicious prank and reported it to the police, but I am so glad now that this person did it. He, whoever he was and whatever his motivation, did me an enormous favour, even if I couldn't see it at the time. I would just leave things be.