after an axillary clearance

I had an axillary seroma, found on ultrasound. It suddenly became palpable about 4 weeks after I finished radiotherapy.  What I could then feel was about the size of an egg. Scared the living daylights out of me. It felt much like you describe, nad it drained once, refilled almost straight away. It didn't cause any problems so  I didn't bother again. It remained palpable for at least a couple of years,  gradually reducing in size. It was still visible on my recent annual checkup, along wth the one at my lumpectomy site, that gives my breast shape  :p.

Get it checked for peace of mind 

Take care