What Causes Pus Filled Scar Post Hysterectomy?

Hi, My aunt has done a hysterctomy on 28th feb 2015 and she has developed a scar above the stiches. She is 59 years old and is having high blood pressure. She checked the scar out but the doctors said that thats normal. Its quite a big scar having hard surface and it has pus also. Doctor has only recommended cleaning the would and it has started turning dry but am wondering what is it.