What Causes Bleeding With Clots From Bed Sores?

My friend was in a coma for about 2 weeks and developed a bed sore about 3.5 inches by 2 inches which was being dressed and healing well however last night he stood up and blood began running down his legs and huge blood clots too. The district nurse came and dressed it again but when he woke this morning his bed was covered in blood and big clots. The gp says it just needs to be redressed regularly but considering it has been fine for 3 weeks and is suddenly bleeding so much we are concerned there is some problem with it, can you offer any advice please?