Hello from JustAnswer.
As you may know, the common symptoms of late onset GBS disease is fever, difficulty feeding, lethargy, irritability, difficulty breathing, or turning blue, also called cyanosis. From your description, she has a mild fever. Although you say she is sleepy, since she will arouse to be eating, smiling, and cooing, she would not be considered lethargic. You describe some feeding difficulty, but some of these symptoms could also be related to the acid reflux that is being treated with the Zantac. However, if these symptoms are worse over the last day, then she may be said to have feeding difficulty.
If the only symptom is the temperature of 100.3, being sleepy without being lethargic, and congested, it would be OK to monitor her for now to see if the symptoms resolve spontaneously. However, if she starts having higher fever, lethargy, worsening feeding difficulties, or any of the other above symptoms, then it would be appropriate for her to be seen.
If I can provide any additional information, please let me know.