A one year old dog cannot digest milk, so don't give her any more of it, because it could end up doing more harm than good. It could contribute to the diarrhea and make her worse, because once a dog is grown, its body no longer produces the enzyme that is needed to digest milk.
Without being examined by a veterinarian, it's impossible to tell what the rash could be from or why her lips appear to be swollen. She could have a staph infection, the belly is a common location for a staph rash to appear. The swollen lips, though, they have me baffled, and I have no idea what could be causing them.
At the very least, until you get her to the vet, make sure she keeps drinking water. When a dog has diarrhea, depending on how severe it is, it's possible for them to dehydrate, so water is very important right now.
It's not uncommon for them to develop what feels somewhat like a knot where the incision is when they are healing, and even though the incision might look all healed on the outside, the healing process for all the layers of tissue and muscle takes a lot longer. Eventually, the incision will smooth out and she will no longer have a lump.
Since she is not eating anything, I would get her to an emergency vet this weekend and not wait for Monday when the regular vet comes in. A dog has to be pretty ill to not eat anything at all, so my best advice is don't mess around with it and have her looked at ASAP before she has time to get worse. Please keep us posted as to how she is doing.
Ghilly
Milk can sometimes disagree with dogs, and cause gastro-intestinal upset. They cannot tolerate lactose.
Looking beyond that possibility, with the other symptoms (which could be allergic symptoms) if she hasn't eaten for a couple of days, and has diarrhea, she could do with seeing the vet. It might be wise to do that right now, because if she gets worse over Christmas, you might be stuck, with the vet's office closed....