Why would she look sad and look away from me?
If you are feeling internal "strife" about something and cannot find the words to express what you are feeling, it may be expressed through our "body language".
If there was some form of "disagreement" between the both of you, she may find it hard to "lock eyes with you' because of the overwhelming "internal feeling". A person may also be carrying "hurt" and does not know how to express it.
Depending on "age", the person could potentially be in love and "longing" for the other and internally "crying" because they are in love with a person they can never have.
Guilt - if the person has done something wrong and does not know how to "come clean".
Disappointment - perhaps the person is so hurt that they can't express into words how much pain they are feeling.
Perhaps the person has a self confidence issue and does not feel "comfortable" to look at one directly in the eyes.
Depending on geographical location, religion and or culture may not permit eye contact between male / female.
From the vague description, it sounds like something happened that was either embarrasing, hurtful, disappointing and other such. It may be directed at you if you're the one she feels that way about if you caused it. If it's someone else that caused it, she knows you understand how she feels. On the other hand, someone might be doing that if they need attention and don't know any other way to communicate that other than recoiling into victim mode. Just depends on what's happening