Suggest Treatment For Swelling In Upper Arm

Hi, my main issue is my swollen left upper arm with no pain on swollen region.This swollen appeared around 2 weeks ako. I have noticed from a month ago, some tension on my left side of my back.My G.P. requested some blood tests including linfoma,D dimer which all test came back to normal. I had my mammography test which also clear. Please help me . What is this? I have no problem with left arm except little heavy and sometimes slightly tightness.