Hi there
Obviously, your doc doesn't think you have a UTI, which is why he didn't use any of those other antibiotics.
He thinks you have Bacterial vaginosis, caused primarily by
Gardnerella vaginalis The treatment of choice for this is metronidazole (Flagyl). However, this is usually characterised by a 'fishy discharge'.
The reason it is happening around your period as your doctor has said, is due to pH changes, which are brought on by the hormonal changes at that time of the month. When the pH in the vagina changes, the normal bacteria are killed off, giving bugs such as gardnerella a chance to grow and cause problems.
Sometimes it's pretty difficult to figure out if you have a UTI or a vaginal infection, because both will sting like crazy when you wee!
Hope you get better soon