I Woke Up This Morning With What Looked Like A

I woke up this morning with what looked like a small pimple on my left arm up by the arm pit I gently messed with it to try to pop the white head and then it made a red line on on my bicep, doesn't go past my bicep or on forearm , just on my bicep, the bump is still red now and the line is not as dark as earlier today but it still shows , what is it?