Hello, Welcome to Healthcaremagic. I had gone through your query and understand your concern. vagina is naturally made to have sex without pain or damage to your vagina, when aroused your vagina doubles in size, muscles relax, and lubrication is produced all to allow for penetration. Chances are the lubrication dried up, this is common particularly when having sex for long periods of time. Friction can cause swelling or even injury of vagina – or vulva, which it sounds like is where you’re describing the burn to be. It not necessarily not to be a infection. As other wounds in the body, Friction or rubbing over the wound may cause pain and burn like sensation. Keep the wound clean with proper washing with soap. This will heal the wound rapidly Watch for any signs of infection like itching, discharge, pus collection. Next time when you have sex make sure to have some lubrication on hand in case your natural lubrication dries up, this is particularly common at certain points in your menstrual cycle. Hope this helps. Please reply for further queries. Wishing you good health.