Anyone diagnosed under 40s?

I started having lower back pain when I was 29 and it's continued on for the last 6 years. Nobody can seem to understand what is wrong.

At first, I was told it was my SI joint. Apparently it had a great deal of arthritus when the doctor went to do the SI injection (I guess he saw this on the xray image at the time of injection). SI injection did not resolve pain.

Second ortho said not likely to be my SI joint that's causing pain because the injection had no effect. He prescribed me Meloxicam and it does help the pain, but I know it's bad to take for long term usage.

Saw rheumatology for psoriatic arthritus (because I have fingernail issues, the (Si/lower back?) joint pain and crackling of knees, elbows etc, and psoraisis). She is undecided if I have that and sent for bloodwork and MRI of SI joint. MRI of spine looks good. The initial bloodwork also looked good. She said it's strange that my pain gets worse with movement and better with rest when usually it's the opposite for those with psoriatic arth and that sitting still usually casues it to be stiff, in pain etc.

Is it possible (or likely) that arthritus just develops very intensely at a younger age and that my pain and psoriasis are just different issues? I am running out of ideas on where to turn or who to talk to.