One of the most probable causes (especially if you haven't changed types of hay or brands of pellets recently) is that she's not eating some (or all) of her cecotropes. This is common in very young rabbits - as my vet says "they've got such a short attention span that they forget to eat them".
Is she producing a lot of normal fecal poops (the hard, dry, round little balls) but periodically leaving some soft, smelly poops that are darker than the normal poops and look like a tiny cluster of grapes? If so, then those are cecotropes (aka "cecals") and you don't need to worry about them. If not, then it's cause for concern.
With adults, you shouldn't ever see cecotropes around. If you do, it's generally an indication that their diet is too rich (too many pellets in most cases) or that they're overweight, arthritic or injured in a way that makes it difficult, painful or impossible for them to bend down and eat the cecotropes as they're excreted. Babies don't always remember or bother to eat them, though (plus they're allowed to have more pellets than adults while they're growing, so they may be producing more than they need).