You been eating too much cheese?  :  )
This should really be called serotonin/norepinephrine syndrome to fit the facts (a lot of the effects blamed on serotonin are due to increased norepinephrine)
Cyproheptadine is available OTC in most places but I never recommend mucking about with anti/histamines because it can shoot your immune system all to rat shit.
What is OTC depends on what place you live in, but Tianeptine is easily available online. Feverfew (the herb) grows open source in many areas and costs nothing (it's a serotonin antagonist). Ginger has antiserotonin properties too.
Best,
AR