It depends on how yellow it is--faintly yellow and normal
smelling is fine, but thick and truly yellow and
foul smelling is not so fine and could be signs of an STD. If by "virgin" you mean no oral, vaginal, or anal sex or rubbing of genitals with someone else ever, though, then an STD is exceedingly unlikely.
You should maybe get those cysts and prolactin levels checked out again, and ask the doctor about the discharge while you're there.