I think its best to take her in and find out. If it is something bleeding or infected inside her and you wait for few days for more blood, it could get much worse or even be too late.
Once again, blood in the stools is always a reason to promptly see a vet.
My Demons only symptom before passing away was green stools. he was active, beautiful, and eating well right up to one hour before his death. Please do not take irregularities in their stools lightly, especially blood. As Kalandra mentioned, it could be the only symptom you will ever get that something is wrong.