I'm assuming by milk you mean your own breastmilk? She probably had allergy to something that you were eating--not the breastmilk itself.
Breastfed babies have runny, often yellow seedy or curdy stools. This is normal. However, formula fed babies stool is more like that of an adult stool. I woudn't be too concerned about the color, however the fact that she's straining and having a hard time pooping leads me to believe that she's probably slightly consipated. Once she gets used to the formula, and having to work harder to pass that stool, she should have an easier time going (mind you, formula fed babies tend to be constipated once in awhile). If she's still going every day though, try not to worry about it.
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