Nipple leaking?

First, I am not pregnant and I've never had a child. I am 23. I have noticed that in just about every bra I have that is of a light enough color, there seems to be small circular stains where my nipples lay in the bra. I wash my bras regularly and have never noticed any sort of discharge from my nipples. The stains seem to look similar to a grease stain and the stains do not wash out... I'm a little worried after seeing this for years and I've never thought of ask before.. does anyone else have this problem? Know what it is or why I'm having this issue? Is it normal?..