I do growled/screamed vocals for a band and for my solo project. I'm guessing your technique's a little off. You need to catch the "grrrrr" using your breathing, and not your voice box. Like push the words up from your voice box but imagine closing off part of your throat to restrict the dry sounds from your voice box and the let the air come from deep in your lungs and that'll project the sound. The way I do it, I can do it for hours on end and all that happens is I get a little dry or out of breath after a while, But with the rush of playing that doesn't affect you at all in rehersals... can just be shit if you're tired and practicing on your own. For now, drink regularly... hydrating fluids that is. And on that subject, often it's harder to do the heavy vox after a heavy night of alcohol. that could be why it hurt your throat.