What Causes Pus Discharge At The Site Of Heart Surgery Inspite Of Taking Ceftum?

Good afternoon sir . My sister aged 28 years she had undergone open heart surgery of VSD on 22/08/2015 now she has been discharged from the hospital .After the discharge she had some infection on the upper stich from there puss is coming out due to this we have consulted the doctor who had done the surgery.he had told us that infection has taken place.Dr has prescribed ceftum 500mg tablet for 5 days .and betadie ointment after taking the tablets still it is draining again we have consulted the doctor .Dr told us that still infection is there you need to take ceftum 500mg tablets further for 5 days morning 1 and evening 1 tablet .and Dr has done a punctre around the infected stich and removed the stich.today morning when we removed the dressing to do a new dressing which was done by doing we saw still there was puss on the dressing pad and little water is also coming out of that stiches.is this tablet is cureable or we have to consult the Dr again.is it serious and what all the precautions we have to take. what we have to do plz suggest us .my email id is YYYY@YYYY