Frozen breastmilk smells?!

[corscors]

I just thawed out bm so my brother could learn how and use the bottle before he babysits next week...When he started feeding, the baby wanted the bottle but kept on making a funny face. I tasted it and realized that it tastes (and smells) fishy. The last time we gave him a bottle was almost 2 months ago and the milk was fine, as it has been every other time my husband has fed him with the bottle. What could be causing this? I'm concerned that my entire stock is contaminated!!!!