The burning you feel is the lactic Acid in your body, your breathing more and having more oxygen move throughout your body. But it does no harm, only some pain.

its probably your muscle tearing. its a good thing if you want muscle cause once it repairs itself, your muscle will be bigger. but if you want a six pack, make sure you do cardio a lot as well to get rid of overall body fat. resistance training alone wont give you a six pack if you still have excess body fat