I have a swelling after dental abscess drainage, what could it be?

Abscess Returning. If you've had an abscess drained, and you still have swelling, it's likely because the abscess is re-formning. Most of the bacteria causing the swelling are coming from within the tooth. Therefore, the tooth itself must be addressed, or the abscess will always return. You need root canal treatment or tooth extraction for a definitive cure. Drainage and antibiotics are only for pain relief.
Could be. The swelling could be residual infection or an area that was not drained via the incision. Another possibility could be swelling due to surgical manipulation of the infected area to reach the loculations or areas where the abscess may be confined.
INFECTION. You had dental abscess and was drained but you developed swelling. Chances are you still have the infection and you need antibiotic and warm compress to the area.