Ah--that's probably it--sorry, my mistake. But yeah, the mirror would do it. I remember taking some shots without realizing there was a mirror above me. Some light bounced 'normally' on the subject, some bounced via the mirror creating a massive line across their face ! The issue you have here (just a shadow) though is hardly noticeable.
In terms of blocking the flash, I just mean that when I'm bouncing from the ceiling (I note you're bouncing behind a little) in portrait mode, it's often the case that the top of the flash head can 'see' the subject and can leave an unsightly shadow behind. Sometimes all you need to eliminate this little bit of direct light is to kind of cup the top of the flash at the front. I keep meaning to add a very short snoot (a bit of tape around the top of the flash creating a bit of a wall, would do it though), though I've been lazy.
I think this link can explain that trick much better: