I have a red spot on my vaginal vestibule. i've been routinely tested for all stds and have tested negative. what could it be?

Hemangioma?: Most bright red spots on the skin, including the vaginal area, are hemangiomas -- a cluster of blood vessels sometimes called a blood vessel mole (but not really a mole of any kind). They are harmless and normal, so leave it alone. No STD causes such a problem. Show it to your health care provider next time you have a routine exam. In the meantime, don't worry about it.