Sore on nipple

Sadie • 20 , first time mommy , due October 20th
I am 18 weeks pregnant and my nipple been very painful for the past 2 days especially when something touches or rubs against it .. well I just notice it has a sore ( looks similar to a pimple ) that looks like its getting bigger by the day .. does anyone know what it is .. or has experienced this before ??