Why does she change the subject sometimes when we are talking?

[DBliss567]

-Maybe she has ADD.

-Maybe the conversation is getting into uncomfortable territory for her.

-Maybe she's bored. (Which could fall into the ADD thing.)

-Or, maybe she's just looking to continue the conversation and that's just her way of doing it.

I'd say it's difficult to speculate a behavior like that without knowing more about her. The fact is that while it's better to understand what's going through her mind, gentle control of conversations is an easy skill to practice and will benefit you and her in the long run. Clear communication is the goal and an aspect of that is keeping conversations on topic and addressing tangents afterwards.