@OP
sounds like your muscles were trying to relax from being worked out...especially if this isn't a chronic problem. Also you stated that your pain is mostly joint and nerve related which is something I wouldn't try a muscle relaxer for but something thats more for inflammation.

I can't answer what happened because you had responses from other muscles that weren't troubling you...to your knowledge. For muscle pain there are a few things I do as alternatives to medication.

If I DID try an alternative med, I'd ask for Valium. I take Valium for muscle relief before bed or half during the day if I'm stiff. As a preventative measure I stretch when I wake up and before sleep. It makes me dizzy so I either do them on the ground or hold onto a wall.

ALTERNATIVES:
hot baths.
icy hot or similar
TENS unit
arnica. thats homeopathic. basically it helps your body release more magnesium.

Luckily it doesn't sound like chronic pain but it does seem like an odd reaction. There are other meds you can take for muscle pain besides muscle relaxers like anti-inflammatories (ibuprofin) or low doses of valium. Best thing to do really is small preventative measures. Swimming is the best thing I can do to keep my muscles working without too much pressure.

I'm interested in seeing how that happened though.