It sounds like you understand the reasons for the additional fluid and electrolytes but elderly patients are more likely to develope extracellular fluid volume excess due to chronic diseases (renal, cardiac, etc.). For scientific rational I would look up homeostasis in the geriatric patient. If your patient does indeed have an infection, this would contribute to her confused behavior (NDX Altered Mental Status). You most closely monitor her response to antibiotic, espechially with regard to renal function. GI function can also be compromised with antibiotics and diarrhea is common.