What Causes Itchiness & Abrasion In The Nipples?

I am a male. I have recently started jogging/running. I faced itchiness on my left nipple during running due to abrasion on the t-shirt when my nipples turn erect. After a while, I noticed my left nipple grew a few small lumps that kind of merges with the nipple when they are erect and they turned dark brown. I have light and pinkish nipples all the time but now a small part is dark brown on the aeola. My left nipple is not itchy but it will if i happen to scratch it or touch it. What possible problem am i facing ? I