Probably my seating position, not at the bike but everywhere else. Plus maybe my scoliosis. I went to an emergency traumatologist and told me it wasn't anything extremely wrong and it looked like only the muscles affected. He told me to get 10 sessions of kinesiotherapy and after that do an X ray. Which was weird cuz why wouldn't I get the X ray first and why an X ray and not an tomography or ecography that actually looks at the muscle. So I went to a kinesiologist that isn't in my medical insurance but which I trust and told me I didn't have anything extremely wrong but I should get a tomography (or something like that) which he told me to wait for the medical-insurance-covered traumatologist to ask so I can get the scan free, and in the meantime gave me three sessions of kinesiotherapy (90 minutes of massages each session). 99% of pain/annoyance went away, but whenever I try to ride my bike again my lumbar muscles starts to feel a bit funny so I get scared and stop.

Last time I measured my bike frame it was the correct size for me. It's a 26" wheel mtb and the seat has always been at the horizontal level as the handlebars.

I went to the bike shop to get a stem extender and now it looks goofy and I don't like how it feels, but I only rode it 20 blocks since I got it. Anyways, the dude told me "ah you should get a 29" wheel bike to solve your problem" which basically gives me the same seat-handlebar-relative-position but it doesn't look goofy.

But anyways, before all this I used to cycle 15 - 20 km each day with a ~5 kg backpack which was not a good idea at all.