It depends on a lot of factors actually! From what you're saying, one of the most prominent reasons I could think about is a cross-contamination that happened during the storage of the frozen breastmilk. While breastmilk can be frozen from up to 3 days after it's been pumped (while keeping it in the inner part of the fridge before getting frozen), it is best to freeze it immediately after. You'd also have to keep good hygiene throughout the process of transferring and freezing your milk - before storing them in an enclosed container AWAY from meat, seafood and other frozen items in the freezer :)
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It is always advisable to give your lo lukewarm milk. If she suffers from upset stomach, see the paediatrician. He will some syrup to stabilise the baby's tummy.