Thats what I guessed from your comments but for that intensity level and given the symptoms you need more carbs.As long as they are being used to replace muscle (glycogen)fuel stores they will not add to fat stores as only when muscle stores are fully saturated will carbs be stored as fat.If you are looking to burn fat during training and want to maintain a low carb approach you should either lower the intensity or duration or both perhaps doing more of an interval routine varying the intensity.