I'm sorry to hear your cat has been having this issue.
When you consider the sneezing along with this new swelling, there are 2 things I would strongly consider.
1) Dental disease. Cats with a mouth abscess or rotten tooth may sneeze and can also develop a swelling under they eye. While these cats may show other symptoms such as bad breath or red gums, this is not always the case. A vet would need to perform a thorough oral exam to check for any issues within the mouth. Though we see dental disease more commonly in older cats, it can happen in absolutely any age.
2) A foreign body. Sometimes, cats will inhale a blade of grass or something similar by mistake and it can get stuck. As the body struggles to break it down it causes irritation, hence the sneezing. Over time, it can cause a local infection (abscess) which may result in a swelling.
There are other potential causes, such as a tumour, allergy or trauma but these would be less likely given the presentation.
It's reassuring that she is otherwise well, but we do need to look into this further. I would suggest a vet check ASAP to look inside her mouth and nose and to assess her all over (checking for any lymph node swelling, fever etc).
If she has dental disease, she will likely need a dental treatment to remove any rotten teeth, as well as antibiotics.
A foreign body might need a sedation and a small camera to find and remove the offending object. Sometimes, the body takes care of the foreign body itself, and medication alone is all that is needed.
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