Toxicity can be caused by lots of things. Stomach flu, any time you are dehyrdrated, being too hot and sweating a lot, exercise without water, seemingly random physiological changes (at least that's what 2 of mine seemed like), not getting enough salt to help keep your fluids in, taking meds that interact with lithium (which includes ibuproferon and naproxen--no NSAIDs on lithium).

My first toxicity started with stomach flu and went on for months because I had an ulcer and so vomiting made sense (I was under a ton of stress) and my levels would come back ok because I was vomiting so frequently. Finally I stayed off work to keep the meds down and came back toxic. The next time I had no cause; I started hallucinating and my psychiatrist sent me to the er pretty sure it was neuro. It wasn't until they had a med student in learning about toxicity that I realized I'd had diarrhea, increased tremors, difficulty walking, speech issues, headaches, all lithium toxicity sx (and also sx of other things) for weeks. The 3rd time I had no warning but just was violently, horribly ill and didn't go to the hospital because I was stubborn about it.

I would look it up and not just rely on my memory. Those are the big ones as I recall but there are sure to be other causes I'm forgetting. After all I had toxicity 3 times, I clearly wasn't great at noticing .