Swelling at incision site

My mother had her right upper lobe removed via VATS last Tuesday.  She came home from the hospital Sunday.  On Monday morning, she notices a walnut-sized lump at the incision site under her arm and some swelling.  By the middle of the day, the swelling had increased 2-3 times.  We called and talked to a nurse that told us it was just fluid and should be fine.  I of course am still worried about it.  Has anyone experieced this and if so, how long did the swelling last?