abcess in colon

Although, I do not have colitis or crohns the proctologist gave me 4 weeks of Antibiotics to treat a abcess that discharges pus with no traces of blood. He said if it does not heel within 4 weeks then a operation would be needed. After a bowel movement in the morning about 1/2 hour after I feel pressure in the colon area and it stays with me for at least 4 hours. By evening I feel okay. The doctor did not have a answer when I asked him if the Antibiotics did not help then 1. try another Antibiotic or 2. maybe it is not bacteria parasite etc. Any answers from anyone would be appreciated.