You're mixing too much epoxy in too small a container. The epoxy curing process is exothermic, which means it gives off heat. If you mix too much in one batch and don't allow the heat to dissipate, it could go into a runaway situation where more heat = faster cure and faster cure = more heat.

BE VERY CAUTIOUS! Burning epoxy fumes are extremely toxic. Mix epoxy in smaller batches and/or in a wider cup in a well ventilated area. If it does start to get too hot, hold your breath and take it outside QUICK. Your lungs and brain cells will thank you for it.