Any incision in the body disrupts normal anatomy. The healing process will ultimately produce reconnections of small arteries and veins, lymph channels, and some minor nerves. Immediately after a tummy tuck, the incision acts as a wall or barrier to drainage and most patients will notice fullness above the scar especially at the end of the day after they have been up and around. Over the first 2 to 6 months, the body will continue to "remodel" the area and produce reconnections of the draining mechanism.
There are some patients who may have persistent swelling and benefit from liposuction of the area to reduce the fullness, however I usually suggest waiting at least 6 months to allow the body to soften the area.