I agree, it very well may be boredom. That and it is getting warmer out now, the ground is cooler & often times they will dig a hole to lay in to cool down.

I say to get a kiddie pool for her and make sure she has plenty of shade so she has somewhere to lay that is cool. And to exercise her a little more often. Between the 2 of those she should start diggin less and maybe stop all together.