Advice?!?!

Someone please, please, please help me! Every time any amount of sperm (little or large doesn’t matter on the amount) get inside my vagina it starts to burn/sting. This happens for about 5-10 minutes after it happened, even after I have a wee and flush it all out! Can someone please tell me if this is a medical issue or does it happen to everyone? My friend suggested maybe an infection but the last time I got sperm in me (apart from the recent time) was at least six months ago so I thought it would’ve cleared up by now if that was it. She also suggested that it could just be the heat of it burning me which is possible but I flush out, what seems like, all of it! I don’t know, I honestly just don’t know what to do. Please give me your honest opinions! Tia ❤️