Got any ideas on this one????

2 1/2 yr old mini white, intact, for the last couple weeks, she has been "Burrowing" as I call it. She will get up in the middle of the night and go under the bed and dig!? It just seems like it is something she has to do and then she's ok!?
Any Ideas?
I thought maybe she was pre-inheat? Never had puppies. She has cats that she can be mean to!?