This could be very serious. Any chance she got into something toxic or was eating something like bones, etc?
Hope you have seen the Vet by now and she ok. Please do let us know
Was the bloody stool bright red or brown? That plays a factor in determining how serious this could be.
Also, call the vet. You don't want to wait if this is the first time this has happened and you're not sure of a treatment, etc.