I very rarely see my nipples, and not with a partner so no-one sees them either! I do breast feel checks regularly though and never felt anything.
However today I happened to glance in the mirror and one of the nipples is a little bit darker (not a lot) and looks like it has a cut right through the middle sorry if that is a bit confusing - bit like a slashed zero
It has always looked slightly like this with a dent in the middle, but more pronounced now. I have googled but can't work out how to phrase it to see if it is something of concern!