Ask your vet - there is a post-op food called Hills AD - and it is LOADED with everything. Also - ask if he would recommend a calcium supplement. (Perhaps some Whiskas cat milk would help, as cats tolerate it better.)

Some tips I found on Google:

Nursing

Pregnant dogs and cats lose weight after giving birth. However, their nutritional needs increase dramatically -- energy needs can be 2 to 3 times their normal adult maintenance requirement, depending on litter size -- in order to produce the milk supply that will support the growth and nourishment of the offspring. Adequate water intake is also important for sufficient milk volume.

To ensure the nursing dog or cat is receiving enough nutrition

* feed a nutrient-dense diet (eg, performance diet, puppy or kitten food)
* without increasing the amount of food offered at a meal, increase the number of meals throughout the day
* Free-choice feed -- offer unlimited access to dry food throughout the day
http://www.peteducation.com/article....&articleid=906 Good and longish article - with some cute pics of nursing moms!

http://www.peteducation.com/category...cls=1&cat=1388 Home page for the above link...I put it here because of ANOTHER cute picture!

Hopefully your vet will have some ideas - and a phone call is cheap!