I made baked cod last night for dinner. I promptly put half of it in the fridge to save for lunch today. Today, I reheated it in the oven … and now, as I’m eating it, it has some faint, strange ammonia odor that wasn’t present last night. Any idea what this is?
Well, it’s been almost eight hours. Whether you are presently alive will be an important piece of evidence if you expect a correct and factual answer from this thread. 
It’s not lutefisk by chance is it? Did you soak it in draino?
Last time I caught an ammonia smell from baked fish was just after I’d just put the first bite in my mouth. I spent the next two days sick, frequently in the bathroom, perhaps in the throes of a case of mild food poisoning. Never again will I eat fish without smelling it thoroughly first.