Heat Cycles = Loss of Weight; New Allergy?

(This post could probably go in other Forums; MOD, please move, if appropriate).
Questions: does a female cat who cycles in and out of Heat rapidly burn a LOT of calories, and therefore cause her to lose weight? Can this hormonal change create new allergies?
Background: we rehomed three cats (now, four) about two months ago from the stray/feral colony I take care of (condo complaints). They are in a friend's spare room in her house who has lots of practical experience fostering ferals (dogs and cats). "Charlie" (renamed Princess Leia) adjusted fairly well, although slowly. Good health, although somewhat slender. Princess is a sweet, affectionate (although cautious) cat. She gets along well with the other cats, especially “Buckley†(neutered, spayed about six weeks ago). She is roughly six months old.
About three to three to four weeks ago, she started going in/out of Heat every couple of days.
We got her spayed as soon as an appointment was available through Alley Cat Rescue, Thursday, March 31, 2011. She was in Heat when she was spayed. Ear Mites were ruled out; tested negative for heartworm, FeLK and FIV.
Post-surgery, she seems okay, somewhat subdued. She is eating, although not a whole lot. Somewhat interested in playing, although hasn't participated.
After surgery, we were shocked by how much weight she has lost. Could going into/out of Heat burn a lot of calories and cause weight loss?
About two weeks ago, we started noticing that she was getting some kind of skin condition under her chin. She scratches a fair amount, especially around the ear canal. Ear mites were ruled out by a vet, and it is not Feline Acne.
To address the skin condition, we are now feeding her (and the other cats in the cattery) dye-free dry food. We are thinking/hoping her dermatitis is from an allergy to red food dye as opposed to a specific grain and/or protein (prevalent in dry and Friskies wet food).
Is it a coincidence that Princess Leia developed a skin condition shortly after she went into Heat? Any other ideas about the condition under her chin.
Thanks for all your help and advice, and sorry for the lengthy post.

Yes, it's hard to keep condition on an unspayed cat, due to the stress and calorie-burning effects of being in heat. Allergies can also be triggered by stress, which may have been because of being brought indoors and living in close quarters with the other cats, or may have been because of being in heat. . .hard to say.

I hope she feels better and put on some weight soon!

Are the cats eating out of plastic dishes? That can cause reactions in some cats, usually manifesting as a rash on the chin.

Two of the three bowls are ceramic or stainless steel; one is plastic.
For the first six weeks, Princess Leia was fine physically--scared out of her mind, but healthy. Weight acceptable (though a bit on the low side); no rash whatsoever.
I'm kind of hoping it was stress; easier to deal with then a food allergy, especially because there are three other cats in the Cattery. Thanks for suggesting that possibility.