little biology/anatomy lesson here. The lymph nodes work in correlation with the nerves and blood vessels. If there is any slowing of either of these mechanisms, you can get swelling. One cannot work without the other. lymph nodes help bring fluids out of the tissue into the blood stream for the body to get rid of. when there's a slowdown the "fluid" backs up into the tissue and can sometimes leak out of the tissue. We see this in the legs more than the arms but it's not unheard of. ESPECIALLY if there's been trauma or sx to the axilla (armpit) or breast area.
Hope this clarifies things.
Also, if this is something that developed overnight or is increasing in size, I would call your doc immediately to rule out infection or other health problems.