Firstly, do you know the context of the message? It could be that the person was talking about someone newly diagnosed (even themselves), and you were mentioned in a supportive, "really got her act together" sort of way.
Secondly, I have no idea which variant you have, but as a diet controlled T2, I cannot see why anyone (in the context of work) needs to know I am diabetic. As a T2, not on medication, I am hardly any more likely to hypo than a non-diabetic person, and other than for that, why would anyone reasonably need to know? Again context.
I would certainly prefer not to be talked about, but in positions I have held, that is unlikely to have been the case.
In your shoes, I would consider speaking to the person who told their colleague and try to get to the bottom of it. Then you could also let them know of your preference for discretion.
I'm sorry you have found it so upsetting.