I hope you are feeling better.
You can absolutely get serotonin syndrome from only one type of SSRI if you take enough. Although sources say it's more common when combining more than one med that effects serotonin levels, I think they mean by this that you can accidentally give yourself serotonin syndrome even if taking the meds at prescribed levels because of how they interact with each other in the body.
When I had serotonin syndrome from varying deliberate SSRI overdoses, symptoms ranged depending on how much I'd taken. In milder form, I had maybe 6-12 hours of spasms, general involuntary muscle twitching, and an upset tummy for a few days.
Larger quantity resulted in the above symptoms for longer duration and more severe (think up to a week+, it was a long time ago) but also body sweats, racing heart and very vivid hallucinations / temporary psychosis, and seizures. Legs wouldn't work and I'd collapse and felt high / drunk for days (but the type of drunk where everything is really fuzzy but you're feeling sick too, as opposed to "happy drunk").
I think it can be worse than that though. I've seen documentation on comas, heart failure, significant kidney damage, though these are much rarer and I suspect also down to individual body chemistry or underlying conditions. But am no doctor so could be talking rubbish.