5 month old shih tzu

Question
I have a 4 1/2 mth shih tzu (Sadie)  during the day while I'm at work I keep her in a play pin with a crate attached.  She normally would use the pee pad, all of a sudden she is chewing up her pee pad.  she has plenty of toys and things to chew on, but out of the blue now she chews on the pad.  What would cause this and how I get her to stop.  While I'm at work she would normally use the pee pad, and when I'm home she goes to the door to go outside.  She rarly has as accident.

Answer
It probably has to do with her maturing.  Laying a piece of expanded metal over it might stop her.  Home centers such as Lowes sell small sections near the nuts and bolts.  

Another solution would be to shut her in the carte and see that she gets a mid day break.  Perhaps a neighbor or professional dog walker could.  Also look into doggy day care.