doxcycyline/ vaginal bleeding/ 1 dose of liquid Floridex (b12) instant reduction in bleeding?

Infections like the ones sexually transmitted do cause spotting or breakthrough bleeding occasionally, so that definitely could be the cause for the bleeding you have expirienced. But antibiotics like Doxycycline are also known to have interaction with hormonal levels in women. I’m sure you’ve heard how being on antibiotics also influences effectiveness of birth control pills acting this way and changing the female hormones. Were you close to your period? Antibiotics also can make your period come early or late, so maybe even that was the case.