What Causes A Large Painless Lump Filled With Fluid Under The Skin?

My boyfriend has a large lump under his skin near the bicep/elbow, the first lump was lower on the forearm, the one on the forearm went away after a couple weeks but now there s a new larger lump near the bicep that feels hard packed fluid under the skin, no pain, no puncture from a bite or like a boil, wondering if his lymph vessel could be swollen due to a bacterial or viral infection, any help would be appreciated