I have a 12 year old DLH Calico. She has been scratching at her chin on and off over the years. I treat her for fleas and worms regularly and have visited the vet on multiple occasions and they don't seem concerned. She has now started scratching at her face and the top of her head that she is bleeding and scabbing. My cats are fed NutriSource dry food. Could this be caused by stress or a food allergy? Even if I have changed her food? Should I consult in a different vet?!
As many people with allergies will tell you - they can get worse if you keep getting in contact with the triggers. Also, allergies or intolerances can develop later in life and don't have to be a 'from birth to death' thing. Some cats can be intolerant or allergic when it comes to grains, potatoes, certain types of protein or the specific thickener that is being used for some wet foods. So yes, allergy might be an option.
When a cat has sores or scratches itself, there are generally these common causes: - Parasites (you accounted for that) - Fungus - I assume the vet took a sample from the wounds to check for that? If not, have him do so of course. - Food allergies. I think this might be most likely right now.
There are allergy tests for cats, but they tend to be unreliable at best, so usually it is advised to test giving pure raw meat to the cat for a while - preferrably a protein it hasn't eaten before, for example horse or goat. This is done for about 4 weeks at least in order to make sure that the immune system of the cat had time to calm down, so to speak. (Preferably, the meat should be fed with a pinch of salt, some blood and a pinch of powdered eggshells (for calcium) to there are at least -some- supplements)
If raw meat isn't an option for you, the only other possible way to do this is feed mono-protein wet food without any grains whatsoever. So for example a wet food with only horse ingredients and ~4 percent of carrots/pumpkin should be fine. No grains, no potatoes! The ingredient list should read f.e. 95% Horse (50% Horsemeat, 20% Horseliver, 20% Horseheart, 5% Horsekidneys, 5% Horsestomachs), 5% Carrots. (and of course calcium, phosphate, taurin etc.) Meaning absolutely no unknown ingredients.
As you start the diet, you can ask your vet to help alleviate your cats symptoms temporarily (only if they are really bad as its not like he will give her sugarpills and all that) by prescribing cortisone. Cortisone is an immune supressant (among other things) and will therefore suppress the reaction of the cats body. Its heavy medication though and should not be given (or stopped) lightly.
If the diet proves that your cat does have food allergy, you can slowly (as in - 1 per month) introduce 1 new ingredient/protein at a time. This will help you figure out what exactly she reacts to (there may be multiple things) and adjust her diet accordingly.
Dry food, by the way, is most commonly a no for allergic/intolerant cats - grains are notorious for that and dry food is just factually mostly made out of that. (and therefore is more suited for chickens than carnivores anyway)
In addition to all of the suggestions posted here, I'd also like to point out that it may be cat acne from her own oils/dander/et cetera from things such as food & water bowls (the most likely culprits) as well as bedding/towels/so on that she frequently sleeps on or rubs against. This is different than being allergic to the material itself; instead, it's the oils/etc from her own fur that rubs onto the material, then is introduced back to her fur and skin when she next approaches the material, leading to clogging pores that may get infected (aka acne.)
Some cats (and dogs!) can get reactions similar to how humans do, i.e. itchy or painful bumps, rashes, pimples, and so on. Since the face is a difficult spot to groom, she may be trying to clean or treat the area with the scratching, much like how sometimes humans just can't stop picking at a stubborn pimple, only making it worse and more inflamed.
Steel/metal dishes are especially guilty of this and need to be washed at a minimum whenever you can visibly see some build up, but more likely every week (or even every 3 or so days). It can also occur with fabric goods, but I would be much more hesitant to wash those since cats are such smell-driven creatures and it may turn a well-loved bed into something she no longer has interest in.
If this and the other suggestions aren't enough to stop it, you may also try just keeping an eye on her and see where she rubs her face in the house. It can be any sort of reaction to cleaners used for certain surfaces like a bath tub, plants in the house that aren't toxic to eat but may still upset her skin, or else that is the location of the cat acne build up.
My cat does something very similar. She has anxiety and has to take medicine for it. If she doesn't have medicine, she overgrooms and scratches so much that her face has sores that are awful. I have to take her to the vet when it happens so it doesn't get infected. If you haven't, I would ask if your cat baby has anxiety.