I feel tension not pain when I do them and I mostly do them with one leg tucked so I don't get extreme forces through my back. I haven't done them in a while though.
Dragon flags are one of those movements to me that seem dangerous. I mean don't get me wrong, If you progress properly and warmup correctly I'm sure you can greatly reduce the risk of Injury. However, you are putting a tremendous amount of pressure on your lower back at the bottom postion. maybe work on bridges, planks and side planks for a while until you feel better. take care bro.