Human mothers can have their stores of essential fatty acids (EFAs) depleted, why not a nursing feline? I am glad she has now found a loving forever home and won't have to have any more kittens or live outdoors.
As always
Mark is spot on in advising not cod liver oil - it is also low in the omega-3 EFAs which are the anti-inflammatory component of fish body oil. Do be aware that cats also require saturated (animal) fat in their diet, please don't substitute one for the other. Also please don't be tempted by flax or other plant oils, they are of limited use even to humans tho Holland & Barrett staff may claim otherwise!
If there is an EFA deficiency fish oil may take a while to have the desired effect.
Might be worth noting that the skin is an organ of excretion, through which waste substances can be eliminated. If an animal has an intolerance or allergy to a foodstuff this may be displayed in skin health. If the itching persists it may be worth trying a change of food: in cats cereals (in most dry foods) and fish are common culprits. Alternatively you might try a different flea treatment next time, there will always be a few cats that react to Frontline as with any med. She could even still be eliminating the dregs of the drugs given during her spay.
My last thought was she might simply be getting too warm, if she has her full winter jacket on then opts to bake next to a radiator ... ? Might be worth daily grooming as if she sheds you might end up with furballs to contend with!