it hurts so bad please help

okay so i need some advice. whenever my SO finishes inside of me it HURTS. it burns. nothing can ease the sensation. i’m pretty sure im not allergic since i have gotten it on my skin else where many times and nothing happened. it’s not from friction since we make sure it’s well lubricated. only hurts when he finishes inside of me. If we have sex everyday it doesn’t hurt as bad. can someone please help? i have no idea what could be causing this. neither of us have STDS.