underarm puffiness

 i have some kind of puffiness in my left arm pit.It's been there for about two months but I just realized it a week ago.It doesn't hurt,.it looks like a bulge from inside my skin. it feels like skin when I touch it . could this be symptom of breast cancer ? I don't see any changes on my breasts ,however my left breast is more sensitive than my right .can someone tell me what this could be ?

   thanks