Stinky Sledgehammer, expired?

My bottle of Sledgehammer (foxfarm) smells different than it did last month, I'm not terribly sure what smell it's supposed to have but it doesn't smell great.

Is this common, or should i get a new bottle?
 
I flush every 2-3 weeks, depending on the setup. The flush before I noticed the smell went fine. I haven't found any resources about how to identify spoiled/expired nutrients, so I figured I might as well start a thread that people can find, for posterity.
 
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