I had a persistent seroma on my mastectomy site. Didn't disperse naturally, didn't disperse with aspirations, and with hindsight I had two mild infections (no pain, flu type symptoms, fever and wooziness) before the really bad infection (pain, swelling etc) that required a week in hospital and surgery to clear out. The ultrasound may hurt but it's sensible, depending on how developed, it may also require some heavy duty antibiotics. But you are doing all you can. I too was using massage to good effect, but don't think I started soon enough to to foil the bad bacteria!! Best wishes for a rapid recovery.