A WBC count of 18000 does point to a bacterial infection, and your doctor adding a second antibiotic after reviewing that report is a deliberate, doctor-directed step, not something to question on your own. Combining antibiotics this way is often exactly what's needed when the first one isn't fully covering the infection.
Next Steps
Take both antibiotics exactly as prescribed and complete the full course, even once the fever settles. Track your temperature twice a day, and check with your treating physician if it hasn't come down within 2 to 3 days.
Health Tips
Seek urgent care if you develop breathlessness, a racing heart, drowsiness, or the fever keeps climbing despite the antibiotics.