Thanks for choosing health care magic for posting your query.
Burning in the vaginal area can have plenty of reasons.
The most common among them are inadequate vaginal lubrication
. That causes problem of burning and pain.
The other cause could be allergy
to the lubricant or any solution that you applied before having sex.
One possibility is that there could be a small tear which is causing pain to you. It may or may not be seen clearly.
And lastly any infection , but then as you say your boyfriend and you are STD
negative then there is less chance that it could be infection.
You need to see a gynecologist who will examine you in detail and help you in coming to a solution.
Hope I am able to answer your concerns.
If you have any further query, I would be glad to help you.
In future if you wish to contact me directly, you can use the below mentioned link:
Wish you good health,
Dr. Srikanth Reddy M.D.