When I was first on lithium I was still diagnosed as MDD and bipolar hadn't really come up at all. I went on lithium because it was effective at reducing suicide risk and boosting ADs. But almost immediately it made it very clear that I was bipolar because suddenly the cycling was much more obvious than it had been. Dose adjustments and getting off SSRIs and onto mood stabilizers helped. But lithium really was the first thing that made everyone certain that I had bipolar. I think my friend who is a psychologist suspected it but otherwise it was a bit of a surprise to everyone but then it became very obvious that it was accurate once people thought about it. So maybe you've got something like that going on?