I doubt it could be 6lbs, but usually when I do heavy cardio, and weigh myself before and after, I loose around 2lbs, but thats not the body fat, thats just the water content. Its only if you burn around 3500 calories that you loose 1lb of body fat. Weight loss, even in the gym is a very slow and gradual thing, it requires alot of patience and ofcourse will power.