Thanks for posting your query.
Persistence of lump after incision & drainage
(I & D) indicates fibrosis
of surrounding tissue around abscess
This type of fibrosis won't decrease after oral antibiotic course.
You should consult with general surgeon
and should go for through check up.
Whatever I & D was done previously, helped in drainage of pus from abscess.
Meanwhile, you can continue with the prescribed medication and wait for few coming days for improvement.
Is size doesn't decreased then go for removal of fibrosed tissue.
Dr. Mayank Bhargava