This article does a pretty good job of explaining what a seroma is:
http://en.wikipedia.org/wiki/Seroma
Seromas ususally consist of the plasma part of blood, whereas lymphedema is actually lymph fluid - not part of the blood at all. While a seroma actually forms a pocket of fluid that will push tissues aside, lymphedema is spread throughout skin, muscle and fat cells of a certain region. Lymph is a different kind of fluid, often containing bacteria, cellular debris and toxins that the lymph system is trying to filter out of the body. That's why lymphedema can be such a serious problem - it's very easy to get infected through an open wound in an extremity or body part that is affected by lymphedema.
The fluid in a seroma will usually reabsorb into tissues over time - and yes, sometimes it takes quite a while for this to happen. Lymph fluid does not reabsorb into tissues. It is meant to drain into lymph nodes, and the wastes it carries are then taken to the kidneys and bowel and excreted from the body. When the lymph nodes are gone, lymph fluid can only be drained naturally by gravity or the massaging action of muscles. That's kind of a catch-22, as too much muscle activity can increase flow of lymph fluid TO an area that is already bogged down by lymph fluid it can't get rid of.
Some drs will recommend low-level heat to help a seroma reabsorb. Heat is actually contraindicated for lymphedema, unless it is accompanied by gravity drainage or active massage, and even then caution must be used because tissue that is overwhelmed with lymphedema can actually be damaged by too much heat.
Coping with a seroma definitely is not fun, but in terms of bodily risk, seromas are temporary situations that in most cases resolve without problems. Lymphedema is a lifelong condition that if left untreated can have some very serious sequellae.
I hope things start going better for you now that the drain is out. Be patient, but remember to keep a close eye on any drainage that you may still be having. If you notice a change in color or a worsening of odor, start to have severe pain or start to run a fever, be sure to let your dr know.
Sincerely,
Tre