You should turn your horse out in a sandier area (in my experiences horses tend to prefer their rolling spots a little dusty) so she can roll when you aren't on her. This would make her less likely to roll under saddle, unless it's a trying-to-get-you-off roll. Anyways, rolling is a dangerous behavior. You may not have gotten hurt, but you could next time. Many serious horse-related injuries are not caused by the fall, but by a horse's weight coming on top of the rider. Catch her before she goes down, when she is starting to slow down and hesitate. Then get loud, get her to leg yield, tap her with a whip, just keep her moving until she isn't going to go down on you.
"The trouble with the world is that the stupid are certain and the intelligent are full of doubt"