IS this a lump or something I should worry about?

A few days ago I was in the shower and I felt something weird in my right inner arm, where my armpit meets my arm. So its not in my armpit, but at the top of my armpit, where my armpit meets my inner tricept. I am not sure if it is a lump, it feels soft to the touch and it doesnt hurt at all, kind of more like a bump than a lump. Feels a bit like muscle. Oddly I cannot feel it if my arm is down, or even only raised halfway. In fact if my arm is down by my side I can't feel anything hard or lumpy at all. I can only feel it when my arm is pretty much fully outstretched over my head. You can't see it and it is not particularly big, but its not on my other arm and I have never noticed it before so I dont know if it is something to be worried about or not. Is it worth going to see a doctor? I do work out quite a lot, could it be some sort of inflamed muscle?