Since she has been on raw for awhile, my first assumption would be that her diet may be too rich. This is almost always an issue with feeding too much OM (kidneys, liver, etc.)
Food sensitivities could be another concern, but if her only symptom is soft stools, then I'd say it's probably just the OM/overly rich diet issue.
Proud parent of Scamper (age 12) and Robin (age 4)
At the rainbow bridge: Slippy, Faramir, Gabriel, Lucky, Ammy