I had the exact same thing happen to me. I think it was because of the way I fired it. I posted pictures and the firing schedule I used on my blog at http://www.fusing101.com
There’s some great information on firing schedules on this site under “tools”, then “firing schedules” here’s the link http://www.fusedglass.org/tools/firing_schedules. I’m very new to fusing, so everything is an experiment to me right now, but I’m having fun.
Best of Luck,