That's a hot motor for Slash, but if you have good ESC and motor Temps then more power to you. About the only thing that will cause a LiPo battery to heat up is if you damage it. That being said, if you are running a a cheaper battery with a low C rating, like 20C or 25C, with a motor that hot you run the risk of enhancing any internal deficiencies in the battery. A good ESC and hot motor will require a lot of current and a low battery has a high internal resistance... High current plus high resistance equals heat. Usually the battery will just limit the current to the ESC slowing the car down, but sometimes the ESC draw is just too much. That is why Novak recommends at least a 30C(I believe) battery for there higher end ESCs like the Havok and Kinetic. I think the GTB could probably fall in that category.

definitely don't use that battery again and make sure to buy or use a good quality high-C battery. Pro-Tek from A-main hobbies has some good quality high-C batteries that don't break the bank.