What Causes Nipples To Have Dark Crust Build Up?

I am a african american female and I have one son who is almost 3. I have had no miscarriages that I know of and I have regular 28 days cycles. My last mentrual was the 15th of may it is the 17th of june and I am still not on my period. My nipples have had a dark crust build up on them for almost 2 weeks. What is wrong? I am about 160 and 5'6 and I am 20 years old.