She may have formed eggs and then resorbed them. Some female igs form eggs very erratically, even only twice in a lifetime. Every time they act gravid, they need to be treated as though gravid.
If you do use sand, make sure it's granules are rounded, and not sharp. Sharp grains pack together making a very dense layer. Rounded grains flow smoothly when dug into. I suggest the size of the grains should be 30-mesh or larger.