I used to have the same problem after my twice-weekly morning 10km run. I was very surprised because my hypos came in the night, which was so long after the exercise. When I spoke to my diabetes team about this, they suggested two things: changing my long-acting insulin dose to the morning rather than the evening, so that the peak of the insulin's effectiveness was during the day; secondly to have something like a banana or a piece of toast before bed. The reason they gave is that when you have done intense exercise, your body recovers at night time when you are resting, so your blood sugars will dip during the night.
Roller derby sounds great!