The only thing I know of that makes nipples redder is breastfeeding. Are you sure they are redder and not darker?

well, i am not breastfeeding! so, it probably means that it is darker. i dont know! but all i know is that they are a little bit sore!

After more research, I found that hormonal changes can cause nipples to darken or be more sensitive.
*Edited to add: a woman normally undergoes hormonal fluctuations in her cycle. These hormone levels tell the body when to release an egg, shed lining, thicken uterine lining...