Is it normal for EBF poop to suddenly change in smell, from benign to truly horrifying?

Yesterday morning, the first diaper I changed smelled suuuper bad, like rotten/sort of fishy and nasty. I thought maybe our diaper pail was stinky, so I washed everything and switched to disposables to try and isolate the stink. It's not our diapers and when a diaper holds only pee it doesn't smell bad, but every time she has pooped since then is just super nasty smelling. Same color as normal, though. Is this something I should be concerned about or a normal part of baby growing older? She's 12 weeks old. Could it be something in my diet? Is it something I should worry about?