Re: Sore nipples... What causes them?
If you're not pregnant, then it's probably hormonal. It should resolve itself in time.
In the meantime, I would suggest icing and advil. Those are some pretty tried and true methods for generic soreness and should help with the nipples too.
I would also check to make sure they are not being rubbed raw by anything. If they exhibit rawness, then I would get some lanolin (the same thing nursing mother use) and maybe go bra-less for a couple days to give them a break.
Naturally if it doesn't resolve itself reasonably soon I would check with your OBGYN.