Assuming this "fatty tumor" is all it is I would unfortunately let the smelly thing drain. It's more of an abscess than anything. I AM NOT A VET but I don't think I would cover it till it's done draining, her licking it will clean it better than you could. Then put some antibiotic ointment on it and if you got some amoxicillin (the human kind even) I would give her a few doses through out the next few days just to be on the safe side.
