I was diagnosed with bipolar the same way as you - after getting manic on an antidepressant. I'm depressed most of the time but looking back at it I did experience hypomania much earlier in my teens and onwards.

When I was diagnosed I asked whether it was just because of the meds and that I wasn't really bipolar. But the doctor said the antidepressants just caused it to surface - it was there all along. So I don't question it especially since some treatments worked well for me.