working out on rest days - am i over-training?

So, technically today was supposed to be the optional stretching day on FB Burn 2. But i felt i had so much energy i did a 4/5 30 min HIIT and core workout to my stretching workout. It was like 3/5 for me even when i did the core exercised with 3 pound weight. Am I over-training? But i still have a lot of energy left and feel like I could probably do another 30 min HIIT (even though i'm not going to). Should i not have done the workout? But K & D always says listen to your body and that's what i did. So it should be fine right?