SSRI toxicity is the same thing as serotonin syndrome which is the body’s reaction to having too much serotonin in the neural synapses of your brain. Serotonin is one of the many neurotransmitters in the brain. Neurotransmitters are how neurons “talk” to each other.
You get serotonin syndrome usually when you take too many drugs that affect serotonin levels.

I do wonder why you experienced this if you were taking your usual dose, unless you were taking other drugs that influence serotonin. These could be other antidepressants, tramadol, codeine, cocaine etc.
Above, Cathy said she experienced it after taking Mucinex DM. “DM” stands for “dextromethorpan” which is a chemical alternative or isomer of codeine. Unlike codeine, it can’t be used for pain, but it will, like codeine, suppress coughs. It isn’t addictive nor does it cause “tolerance” which is where you have to increase the dose of a drug to get the same effect. So if Cathy took the Mucinex DM with other serotonin influencing drugs, that might have caused it. Sensitivities to serotonin probably vary, so a dose that causes symptoms in one person may not affect another person.
The treatment is “supportive”—IV fluids and benzodiazepines for the agitation and tremors. Benzos include drugs like Valium, Xanax, Ativan etc.