6 year old daughter talks negatively about herself?

Occasionally my 6 year old daughter will say things like "I don't like myself." "I don't like who I am." "I'm nothing." "I'm stupid." Etc.
Her dad and I are divorced and she has visitation every other weekend. It seems she says these things moreso when she returns from his home. 
We don't say anything like that about her or around her when she's with us and I'd like to think her father doesn't as well. 
Anyone experienced this?
A little note. Her father is an extreme narcissist. He is controlling and manipulative so naturally I'm worried he'll act that way towards our daughter. 
I'm hoping he's not though and that maybe this is normal behavior for a 6 year old? 
I try to speak to her about why she's feeling this way but her response is usually vague and she can't explain herself much. Just "I don't know." Could it be frustration? 
She's otherwise a happy kid. A little sensitive and emotional at times but no negative behavior or anything?