What was your diet today? Your food intake is everything when working out. It's the difference between bulking, burning fat, and toning. Well, that and fitness level. You need to have enough energy to fuel your workouts. If you have complex carbs or too much sodium, your body may not have enough energy to fuel the fitness rigor. So, make sure you're eating foods that can give your body the fuel to continue working at the pace you are.
Usually a good stretch before a workout will help