Doxycycline is a tetracycline. It is normally used to treat a bacterial infection. Bacterial infections are not STDs. The most common one is bacterial vaginosis (BV). Is that what you have? It's normally got a very distinct foul fishy smell. It can also have a milky discharge that despite it's thin milky look, does not run like milk.
A bacterial infection can be difficult to get rid of. Improved hygiene is normally needed. No more sex without a condom as the semen provokes the bacteria, no more douching or any scented things, wiping from the front to the back on the back to keep anal bacteria out of your vulva,keeping everything down there very dry and airy.
Don't just take the treatment from your doctor. Find out what they are treating, how you can help, what to do if the antibiotics does not work. It is your body, do your homework before leaving the doctor's rooms.