Similar issues with my left knee. Had the MRI last week and see the doc this Tuesday. He’s leaning towards a minuscus tear, which I think is preferable to arthritis.

I've had bad knees all my life. Bone on bone. I've very cleverly exacerbated the problem with basketball, running, and martial arts, over the decades. Glucosamine and chondroitin help. I've had a number of cortisone injections. They are wonderful.

Surprised they didn't inject your knee when you went to the er.
I get my rt knee and left ankle injected about every 4 months. It helps for a short time and really calms things down.
Also take a Glucosamine supplement daily. You think it doesn't help until you stop taking it for awhile.

If you're going to get the injection, ask that they load morphine at the front end of the injection so that as the needle goes in, they can inject some of the morphine and numb it. I had that done with cortisone injections for my plantar fascitis.

We spend most of our waking hours sitting down, with our knees bent and they're not working. Then we wonder why they all of a sudden ache. I had an episode of that one time for a long while. Could not walk up and down the stairs from the pain. My fix? The next house we bought is just a single level with no stairs. Doc said I have rheumatoid arthritis in the knees.

Actually, I think I walk more now than when I was still working at a desk.