What Causes A Hard Lymph Node In The Armpit And Reddish Infection?

I have a single, painless hard lymph node in my right armpit the diameter of a pea. I don't have any other symptoms (fever, fatigue, nausea, etc). But sometimes my other (left) armpit also feels a little tender / achey - other times not at all. The swollen lymph-node in my right armpit happened quite suddenly ( 1 - 2 days), and it coincided with a pus-producing and reddened skin infection on the right side of my ribcage - about 4 inches above my belly button, with the affected area about the size of a quarter (coin). Is this related? Do you recommend that I see a doctor?