Boxer Leaving small rice-like debis in bed

Question
My 4-month old boxer has been leaving small, almost rice-looking debris in the bed after sleeping for a few hours.  This has been going on for a couple of months.  At first I thought some sort of insect shell or egg larvae, or even residue from her nails, but it doesn't seem to be any of these things.  She has been tested for worms, and is negative.  Have you heard of anything like this?  If so, what can it be?

Answer
Keith,

Quite honestly, my friend, I've heard of nothing like this nor have I experienced it.  Before you said she had been tested for worms, that was going to be my suggestion. She's only 12 weeks old, when was she tested for worms?  When you brought her home?  Is it possible she's gotten worms since?

I'm stumped.  Forgive me for asking the obvious, you're sure it's from her? She's not getting into anything outside?  Have you tried brushing her before she's permitted in your bed to see if there's anything caught up in her fur?  Go over her really well, check out her skin, see if there are any sores or flakes.

I'm stumped.  Any way you can take a photo?

My e-mail is Nicole@boxerdogplanet.com