Yeah, it's pretty common to question your diagnosis. I've done that several times. Still do.

A few weeks ago, I was questioning whether or not I actually got depression, or if it was all in my head. Now I'm currently depressed, so I know it's not in my head. But I'm sure I'll question the depression piece again. I seem to question it when I'm not depressed, but then I end up depressed and I'm reminded that it's not in my head. Rinse and repeat!