Severe outside vaginal burning, burning on the top of legs and when bladder starts to get full it hurts what could it be all std tests negative?

Yeast Infection: There is no std that has this as it's symptoms. It sounds to me like a simple yeast infection.

Possible Vulvodynia: Your symptoms may be related to a variant(s) of chronic pelvic pain syndrome known as vulvodynia and interstitial cystitis (painful bladder syndrome) which include the symptoms you describe. These are complex, but not uncommon, conditions that are very often underdiagnosed, misdiagnosed, and undertreated in women. I would encourage you to read about them and make appt with a knowledgeable gyne md.