Hi everyone, I currently own a 1 year old yorkie

. I adopted her when she was 3 months old from a girl who unfortunately purchased her a petstore and decided that after 2 weeks she had no time for....
She has been healthy up until now, she has localized mange(around her eyes). I have changed her food to Natural Balance Duck and Potato after reading alot of good reviews, but after being a week on it she has developed constipation. This evening I noticed her having a difficult time having a bowel movement, she was almost like in pain. It took her awhile and she had a bm but all she does is rub her bottom on any surface. Could this be the food I am giving her now?? She use to be on Eukanuba.
Advice..thanks