I am sorry Yodie has this trouble.
This is called an aural hematoma. It is a result of the cat violently scratching or shaking her head.
A vet can drain the fluid but it often comes back if the itching continues. If you do nothing, it can resolve on its own but the ear may have a cauliflower appearance.
In the meantime you can give her an antihistamine:
onion or garlic.
Give it every 24 hours. It is very tiny so she will not detect it
Any of these are also acceptable:
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice a day or
Claritin (Loratadine) at a dose of 2.5-5 mg/cat or
Zyrtec (cetirizine) 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Please let me know how she is doing. Just reply here or if you prefer, we can speak on the phone.
Please be so kind as to rate my answer with positive feedback, as that is the only way I receive recognition from the site for my assistance. Thank you!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed and I will always be here for you and your kitty.