What Causes The Outbreak Of Reddish Lumps On Chest, Armpits And Breast?

I am 25 and I had a a couple of red bumps breakout under my arm but now they are spread across my chest, armpit, and under my breast. Some of the bumps are now darkened and have scabs but others keep appearing. I don t know what they are but I ve had them for about a week and none of them have gone away.