@Sidereal - one 100-mg dose of 5-HTP - not serotonin itself.

This is one pill from a respected brand - Jarrow, I think. After reading
@caledonia's (er... I think it was Caledonia's?) recommendation to sometimes take a pill and divvy it up into 1/125 doses, this was probably high for someone like us, but I didn't know that at the time and took the recommended dose.
I had the same reaction to an herbal blend containing
Withania somnifera, which is serotinergic. I had what is considered a 'normal' dose of that as well: just a few grams of powder, and it's typically very benign. I even had it before getting chronically ill and nothing happened to me.
Finally, I get an 'echo' of the same symptoms if I have dark chocolate. Ghostly pressure at the side of my head, increased dizziness and elevated HR.
My serotonin is quite low normally (naturally?) and it REALLY doesn't seem to like an unexpected jump.
-J