Using someone else's breast milk?

So I'm severely anemic, and can produce any milk. My baby is now a month old, and I've been giving him half formula and half breast milk. The breast milk is coming from his aunt, she had a baby shortly after I did and is producing a ton! So she's been giving me the milk she pumps. I'm not sure on people's thoughts on it, honestly it's still really weird to me. I know breast milk is the best and I was devastated I wasn't producing anything. The first few days I've given him the milk he did great! And he's become extremely fussy ever since, idk if it's because of her milk or what.. I don't really know what to do. He's up fussing 80% of the day, doesn't sleep much, but he is sleeping like 7 hours straight at night. I'm just, lost right now.