You didnt provide alot of info on your burning so Im just going to take a guess.. It is prob a build up of lactic acid in your quads.. It happens when your using your anaerobic system and your creatine phosphate system is out of gas.. The next system that takes over produces lactic acid but the body can only tolerate so much... Thats when you feel the burn...
That may not be it but with the info its my best guess