What Causes Black Lines On Underarm After Recovering From Spider Bites?

I got 3 spider bites on Sunday i put baking soda on the bites and it brought down the swelling however i could see a few black lines that appeared to be the posion. now it has moved to my underarm pit and it is a blackish color. do I need to see a Dr. I saw one yesterday but it had not traveled to my arm pit then. (i'm at work)