a swab will check for any infection e.g. mrsa. this sounds like a complication that can happen after any surgery. possibly poor circulation in that area for some reason and can take a long time to clear, it has to heal from the inside to the surface that's why it needs packing. keep as active as possible and ask at your health food store about supplements to aid the healing process
I don't know what causes it, and I am not trying to scare you, but better safe than sorry, and from what you just posted, it's time to go to the doctor.