I too get it where it hurts to sing. I'm really sorry that I don't have too much knowledge as to why. Although I am prone to upper respiratory type infections.
But my old singing teacher once suggested a period of total rest for a month or so (as in don't use your voice at all). So that might be worth a try. Give your voice some time of to recover type thing. And apparently whispering is really bad for your voice so try not to do that during the rest period!
Sorry I couldn't be of more help but I just wanted to reassure you that you're not the only one that gets this problem
Stan