Impetigo on buttocks and anus? Please help!?

Hello. This is my third breakout of what the doctor class impetigo in the past year. For me, it happens about every 4-5 months. The area I have it is right at the top of my butt crack and then two spots on my butt cheek. I never knew what it was until the dr. prescribed mupirocin 2% and i read the information (its a cream). Anyway, after reading about it I'm scared that i'm going to get a staph infection or MRSA. It starts as like two red bumps that look like cuts and then i get two more on my cheek and thats about it. Its extremely itchy, but I dont itch it. Usually i just let it be, but it was soo itchy this time I called. I put the med. on last night and when I awoke this morning and looked at the area(the best I could see in that area) they bumps had turned a purple color. Is this normal? I can't find much information online about impetigo being on / near the butt crack...Thank you.