Burning??

We decided two days ago that we were going to have him finish inside me. But after he came it started to burn pretty badly. I assumed he just scratched me because we did do some rough foreplay and there was a little blood. But we had sex again and he finished the same way and it still burned. This time a little worse than the first. What could be wrong? Am I allergic to his semen? Is he just really acidic? Is it the possible scratch that I might have?  An STD or STI? Help please