My frozen milk smells weird but isn't off (aparently)?

It is very normal. I think it's a reaction from the combination of plastic and freezing. I don't think it's bitter enough to be a problem for the baby and it's totally safe to use. What I do is just fridge it if I know it will be used in the next couple of days. You can check out kellymom.com. I pretty sure you can find info about that kind of thing there. Hope that helps.