reusing left over breast milk after baby finishes bottle?

Aryana • Due November 11th , 2016 ♡♡
I just recently started pumping and was wondering if it's okay to reuse the breast milk my baby doesn't drink during her feedings ? I usually heat the milk up in a pan gradually starting with warm water then getting hotter until milk is warm.. I fed her 2.5 ounces she only ate 2. Really don't want to throw milk away like I did last night..baby is 2 weeks old.