Can a seroma disappear with exercise even though you are supposed to decrease activity when you have one?

There is no direct link between exercise and seroma resolution. Sounds like the aspirations were effective the skin flap scarred into place and the potential space is now closed.

Seromas are unpredictable creatures. Traditionally we treat them with activity limitation, compression and aspiration. However, on occasion and often times due to frustration with persistence, patients may return to normal activity and the seroma may spontaneously resolve. Unfortunately, it is not a consistent finding. Glad to hear it disappeared.

Seroma can behave erratically. You're right that the lymphatics probably reduced the fluid enough that the seroma resolved. Typically, we want our patients to reduce excessive movement and use some compression until the seroma goes away. The fluid resorbs and the tissue scars to eliminate the potential space. In your case, it seems that your body did the right thing even though you exercised. I wouldn't suggest other patients with seromas exercise to eliminate seromas, but you can't argue with success. Odds are that you were well along the road to seroma correction. Congratulations.