Help any advice?

Hello, my 21 year old cat has in the past 2 weeks started to show her age
I noticed about 2 weeks ago she wasnt herself ,sleeping all the time and lethargic,then a swelling appeared in her neck area below her chin. I took her to the vets who treated her with antibiotics and anti inflammatory injections. The swelling went away the day after her first trip to the vet ,on her second visit she was given another 2 injections, as i said swelling gone completely,but she still seems to be sleeping all the time.
Her appetite is not as good ,she is eating but very small amounts, on the vets recommendation i am feeding her Hills prescription diet ,which is a high protein and high calorie food ,she is getting through about one tin a day,two weeks ago i would be going through 3 sachets of wet food a day and some dry food. Is her age just catching up with her ? or does anybody recognise any of this as another underlying condition ?
She isnt hiding away, when i stroke her she responds and purrs , she is using her litter box all normal there, she is following all her normal routines just much slower ! sorry to go on just so worried

Has she had a full blood work-up done?
It's possible that it's just old age catching up, but there could be something underlying.

Before my old gal was diagnosed with CRF, I noticed that she was losing weight, muscle mass to be exact.
I understand that just as with humans, the internal workings are not as efficient as they were in youth.
If you haven't had a full senior panel done, you should get one done.

Thanks for your reply had the tests done worst fears confirmed ,she had a cancerous tumour in her throat ,which was causing her to have difficulty breathing and build up of fluid in her lungs, really didnt want to put her through invasive surgery at 21 years and so weak the vet really didnt think she would survive . So made her last weeks as comfortable as possible,but 2 days ago had to make that awful decision that most pet owners have to make at some point ,she went to sleep in my arms, very peaceful . I miss her so much but she had a long and happy life ,and will live on forever in my heart .

I am so sorry for your loss - you gave her so much love, and the gift of freedom from pain while feeling your caring and warmth right up to the end.

I am so sorry to hear of this, my deepest condolences, losing a friend you've had for so long isn't easy.
I am sure though that she knows how much you loved her to the end.

Not that it's any consolation, but you should be happy, knowing that you kept her healthy and loved for 21 years, not many people can say the same.