Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Carmella.
I'm sorry to hear that she has been so uncomfortable since meeting a new dog.
While fleas were a reasonable assumption they aren't the only parasite that can make a dog itchy and depending upon which flea control product you used some are less effective then others and some can be very irritating to the skin.
My immediate thought would be that she picked up a parasite called sarcoptic mange which can produce intense itchiness and is not treatable with over the counter flea products. Treatment is with ivermectin, or a prescription topicals available through your veterinarian called Revolution or Advocate.
The other possibility is that she is sensitive to the topicals that you applied and that sensitivity/allergic type reaction has caused her to be even more itchy. If you used Hartz or Sargents products I recommend bathing her in Dawn dishwashing soap to remove them then using an oatmeal conditioner to soothe her skin.
Ideally she would see her veterinarian to make sure she doesn't have Sarcoptic mites as they are contagious to other mammals, including you, but in the meantime in case this is an allergic type reaction and to soothe the reaction from any insect bites you can try:
1)Benadryl (diphenhydramine only don't use the combination products with decongestants or acetaminophen as they can be toxic for dogs) at 1mg to 2mg per pound or one 25mg capsule per 25 pounds of dog orally every 8 hours.
OR 2)Claritin (loratidine only NOT Claritin-d) at 5mg per 25 pound dog once or twice daily.
OR 3)Hydroxyzine at 1mg per pound orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
Some dogs do better on one antihistamine rather than another so I've given you several to choose from.
Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These
side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products that have an anti-inflammatory effect and work synergistically with antihistamines. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight.
Best of luck with your girl, please let me know if you have any further questions.