Think of your vagina as a fertile field....and the antibiotics as a flamethrower.
If you take a flamethrower to a field, and don't plant anything, weeds will quickly grow.
The key is planting the right bacteria, and guarding against the wrong ones...NOT in a one step solution of burning everything in the field.
Ask a good GYN about what you can do to promote beneficial flora (bacteria)
Also, pay attention to your clothing, the detergent you use, and the water you wash in/bathe in. Your plumbing may not be clean because...well, your home's plumbing might be dirty.
(swimming may also be a problem)
and as your GYN may have stated...no undies+ skirt is best, cotton undies is good, and synthetic undies are trouble.