X ray diagnosed osteoarthritis

I'm only in my 20s I'm not a runner, I'm not obese / over weight. I am autistic and have been a tip toe walker (when shoes off) since I was a baby. I've had no therapy with regards this and I don't even notice I do it.

This past few months the pain in my leg has been quite bad, my right knee is quite visibly swollen and it's only getting bigger.

I'm allergic to anti inflammatories (make my eyes and lips swell as well as a rash etc)

I'm so confused as to why this is happening to me. My stairs were the first thing to cause my trouble. Now I have to go down the stairs one step at a time sort of side ways.

Long story short I'm waiting on an MRI. I live in Ireland and the waiting lists are super long.

If they can already diagnose osteo arthritis from an x ray then it means this issue hasn't just started, right? Like if it's showing on my bones then my knee must be pretty advanced already..

Could this be a misdiagnosis? Why would someone so young who hasn't had any injuries or active in sports etc be struggling like this?

Maybe it's more common than I know.. I've been just trying to stay as active as I can and just keep moving because the last thing I need is to stop, right??