Hi, thanks for using healthcare magic
You are right in implying that antibiotics can cause yeast infections
in some women but there is no certainty that one will occur.
Since the culture did not detect any yeast infection or any bacteria, your doctor is suggesting the use the medication mentioned to cover possible bacteria. Metronidazole
and doxycycline treat different bacteria and the hope is that with combined use , your problem will resolve.
To reduce the potential gastrointestinal side effects you can consider the use probiotics. These can be bought and used on their own or you can use yogurts with probiotics.
Probiotics have been shown to help with gastrointestinal issues and it is also thought that they help some women with repeated infections.
I hope this helps