What Causes Hard Bump With Pus Under The Breast?

I have a bump that will appear under my breast and the area around the bump will start to get hard after a couple of days the bump will get bigger and turn red. Then a day or two later it will have puss in it. But it s not white fully it has a greenish color to it once I get out the puss it heals up and goes away. But I don t know what it s called and how to prevent it from coming back. I ve read what other people have said who have some of the same symptoms as me and came up with sebaceous cyst but my bump doesn t get that big. It s only the size of a pea or smaller. I just need to know what it is and how to stop it from coming back.