There are a few things you can do to stop your bum from hurting when cycling. First, make sure that your bike seat is at the right height. If it's too low, you'll be putting more pressure on your sit bones, which will cause pain. Second, try wearing padded cycling shorts or using a gel seat cover. These will help cushion your bum and reduce friction. Finally, make sure you're taking regular breaks to stretch and move around. If you're sitting in the same position for too long, you'll start to feel pain.