Please explain why does it burn during and after i have sex?

Lack of lubrication: Many reasons for your symptoms. I suspect most likely would be lack of lubrication. Try using water-based lubricant next time. As long as burning sensation goes away in hours to days, no worries. But if it persists or occurs w/voiding, then you need to be evaluated. Even more concerning if you engage in unprotected sex which puts you at great risk for sexually transmitted diseases/infections.