Small sore on the edge of my vagina?

I've been having unprotected sex with my husband for about 6 months now (we were each others first on our wedding night.) and a few days ago I developed a little sore and the end of my vagina, it only hurts when it is touched. He examined it for me because obviously I can't look for myself and he said it looked like a sore. I have not gone to the doctor yet, I just need to know what it is and if it might be an STD. I put vapor rub on it and it helped a little because the next day it didn't hurt at all, but I didn't put it on today and it started to hurt again. Could it be some sort of infection? Can I go to the nearest pharmacy and buy some sort of cream or lotion for it? Do I seek medical attention? No he has not cheated, he is very faithful, as am I.