I've been stung by my long spine urchin many times, I finally decided to sell him. The last time that I got stung it hurt SO bad! Mine also left a black spot, I soaked it in warm water for an hour. I also had someone try to dig the spine out, only to find out that there are times that the black spot is only the dye from the spine. I can empathize with the pain, mine definitely brought me to my knees.
I would take the advise of getting your tetanus shot up to date if it isn't already. I paid a visit to the doctor to make sure I didn't have any reaction.
Good luck!