The question is when does evil little girl get into your Head? because I have worked out that Talana is who she says she is at first and gives you dragon powers. This is because one) evil girl does not have dragon powers and therefore can not give them to you, two) evil girl is in Hall of Echoes and can not physically reach out to you, in dragon form or not, otherwise she would just get Damian to come and get her, and I am sure that Damian could figure out how to get to the hall of echos, as he can enter dragon tower freeing, destroy any town he wants and force the Azeroth guy to give him the scroll.
So I have worked out that she gets in your head just after you get dragon powers and as the game says she gets in due to your temporary weakness and lodges like a tick.
OR
second possibility is that she was already lodged in Talana's head and with Talana's powers she comes over to you
I think that it's the first case - She gets into your head after the vision of your first flight as a dragon ends, but before Damian wakes you up. That's the only thing which makes sense to me.
