Our 4 Mth Old Granddaughter Started Passing Mucousy Bloody Stool

Our 4 mth old granddaughter started passing mucousy bloody stool 4 days ago. 5 diapers. No fever, no pain. She is nursing and appears to be eating well. Saw ER Dr in Menomonie, WI Friday night who didn't exam her. Just said it was probably a bacterial infection and gave her parents a script for an antibiotic. She hasn't had any further episodes until this evening. She saw her Fam Practioner on Monday. He was concerned if it happened again as it's not normal. Her mother was wondering if she should take her to the ER tonight, or wait until morning to see if she can see her regular physician? Is there any time frame for culturing the stool for abnormalities? Should they save the diaper?