My Female Dog Was Recently Fixed And Now She Is Vomiting Yellow Foam What Could Be Wrong?

Have your dog examined by a veterinarian to make sure she is
not having complications from her surgery. 
Typically in the 5-7 days following abdominal surgery is when
complications are seen.  She may just be
painful from her surgery and need more pain medication.  Pain can definitely trigger vomiting.  This is very common especially in the 1-4
days after a spay.

She needs to be checked to make sure she is not having any
internal bleeding or has developed an infection.  Post-operative infections are most commonly
seen along the incision line and not in the abdominal cavity.

In the few days after a spay most dogs have a decreased
energy--this is normal.  They are not
getting into as much mischief as a normal puppy.  It is much less likely that she has eaten
something that has made her vomit or gotten into a "poison".

Your veterinarian can make sure there is no serious cause
for the vomiting then treat appropriately with pain medication or anti-vomiting
medications.

Thanks for the answer, I am going through a similar situation with my dog and I am quite worried. She is 6 months old and we got her 4/21/09. Her stools were dark, but I assumed she was settling into her new home with new food. We took her to the vet for shots, spay and check up Friday 4/24/09. She has not eaten a thing since. She is vomiting bile. She is very tired. No puss, swelling or blood around her incision site. She is drinking and urinating fine. But I was just really worried because she is avoiding food and vomiting bile. I took her to the vet today 4/28/09. He ran blood and her white blood count was fine. She was not dehydrated. She didnt have a fever. He just thinks its taking some time for her recover. I just feel so sorry for her and want her well. It tears me up to see her so sad.