You have lost a LOT of weight in a very short time. All of your muscles and connective tissue needs to snap into place. I have been losing weight very, very slowly and it impacts my performance a lot. I think you just need to go very slow on the running until your body has adapted to the lower weight. You aren't just losing fat - you are losing minerals, blood, muscle, etc. And you have lost a LOT very quickly. You are just depleted for now until your body adjusts to the lower weight.