paper cut like cut/sore on my inner vulver almost right on t

I was having sex about 2 times a day and one night we were having sex in the shower and the day after I noticed a sore on the inner of my vaginal lip, it looks like a big paper cut and the skin around it is raw and red. I went to the doctor and they tested me for STD's and found nothing. So they gave me cream to put on there to heal it. Just as I thought it had healed I had sex again and broke the sore open again. I don't know what to do or what I should put on there to heal it. I think it might have something to do with my partner having a large member and I am small. Along with his hairs rubbing me. I know I am not dry becuase I have lots of wettness or lube when we have sex so thats not it. Please help, I really want this to go away. Medicine to put on it or what you think the cause is? Thank you.