Randy, I had a similar problem with some of the rosewoods. I found a couple things help.
Wear gloves, wear respirator, DON'T sweat, get a dust collecting setup of some kind right where you're sanding.
It sounds silly, but sometimes you can get a specific rash like on the neck, etc even from exposure that isnt on the neck.
Some people have a similar reaction to medication where every time they take pill x they get a rash on their forehead or something equally bizarre. So it could be from breathing it, or some other contact.
If you have more work to do on the bow, my advice is get the most protection you can while you work with it, immediately take a shower after and put your clothes directly into the wash, and just immediately get some anti-itch cream on the neck and start popping allergy pills! Doing all this is the only way I can work with cocobolo.