Frozen Pumped Milk Making My Daughter Sick

My daughter (7mo) is EBF and has never really taken a bottle. But I've pumped off and on just to have some in case of an emergency and to use in her oatmeal. I have it stored in our chest freezer in the garage. Since she started solids, I had fed her oatmeal made with frozen milk a handful of times with no problems. Then a month ago, I fed her oatmeal made with frozen milk from December and then put her to bed. A few hours later, she woke up crying and vomiting. Not just spitting up - vomiting repeatedly. She threw up a few times over the course of five minutes, then was fine and didn't throw up again. It was the first time she had eaten solids so close to bed time, and she ate kind of a lot, so we chalked it up to her stomach not being ready for that. So I didn't give her oatmeal again for two weeks, then gave her some with more frozen pumped milk (I smelled and tasted it first, and it seemed fine.) Same deal. Vomited multiple times but then was fine. I have since tried the oatmeal mixed with both formula and water, and she has not thrown up. So... I'm pretty sure the milk had gone bad somehow. I hate to throw away the rest of the milk (only about 20-30 ounces, but still!) but I don't want her to keep getting sick! Any thoughts?? Is there any way to check to make sure it's not bad without just feeding her and seeing if she gets sick (which I obviously don't want to do)? Am I doing something wrong storing it? I'm using the breastmilk storage bags and keeping the milk all together in a container in the bottom of the deep freezer. This is so frustrating!!