First off, chemicals do not set in your joints unless: 1) You have osteoarthritis 2)You're a chronic drinker and/or smoker and/or 3) You have large chemical/toxin intake level. If you're older (like around 45), then i could be vertigo or something like that. It's more common. If you're overweight, you could have had nerve compression or you cut off your circulation causing it to rush all at once once the compression was released causing a passing out sensation.
Secondly, if would depend on how long you were sitting down for. If you were sitting down CROSSED LEGGED for more than half an hour, this could be the issue.
And the last part would be, it could be depletion. I got it a lot when I first started practicing magic. Especially on the ones the don't work and/or were out of my range. When I first tried to do a summoning spell (which didn't work because whoever wrote it didn't know anything about summoning and what it required) I literally couldn't walk. I fell down and I wasn't even sitting during the ritual. Your energy is still being expelled even if the spell isn't going to work. That's why it's best to just start easy and read as much as possible before you start practicing so you won't be foolish.
I'm not claiming these as fact (except for my personal experience). I'm simply stating what it could or could not be.