I usually do the Epsom Salt bath thing. And stiffness and pain usually only kick in the next day.
Sounds like a pretty bad fall, and a friesian is NOT a small horse!
I'd rather you go get things checked out. A jar like that can cause damage, and you don't want to wait a few days to find that out.
When you try avoid a fall, your muscles tense and you do get slightly more hurt.
Please don't down-play the accident; I've fallen off plenty more times tan I can remember, including off racehorses, and I haven't had symptoms like that.