Scratchy undies?

ive been having 'stings' when I shower (I have sensitive skin so I use warm water to cleanse so I know it's not soap or anything) on my labia majora and minora, like as if i have tiny paper cuts on them. I went to the docs and they said they don't see anything out of the ordinary no infection or std. And sometimes it gets a little itchy.the only thing I think could have caused it was About 5 days ago I went on a bike ride and the seat was pretty hard and I was wearing hanes cotton undies and that they could have maybe scratched my skin.. has anyone one had something like this? Or know how long it may take to heal or anything I could use to help soothe the skin?