First of all here's a blend file from OneDrive (https://1drv.ms/u/s!Ai6fbuigY-m32nU70l8Nsm3Gr63Z?e=Ixkf44) that has my mesh model and rig that I made myself.
Yesterday I created that mesh, then I created a rig and then applied rig to mesh first selecting my mesh in object mode, then rig and then pressing ctrl + p and selecting automatic weights.
Yesterday it worked in a way I was excepting it to work. If I rotated leg bone in Pose mode mesh leg moved with it while rest of the body remained still.
Today I continued working. Made all kind of stuff like applying materials, fixing something etc. I also removed that rig and created a new one using technique I learned from YouTube that if you name bones certain ways (like bone.l) then you can symmetrize them so that you don't need to do other half again.
I can't pin point exact moment I broke something but now if I apply that rig to mesh and try to move, for example leg bone, the whole mesh bends like crazy.
I have done much Googling and tried something like applying transformations, fixing normals and all kinds of stuff that I don't remember. I also updated my Blender to newest version so I'm now using 3.6 LTS.
People say automatic weights are, well, automatic, so they won't always work but I refuse to believe (for now) that my simple mesh would be something it couldn't handle. Especially because it was working yesterday.
So if any of you with better understanding could open my file and try to apply rig to mesh and then for example just rotate leg 90 degrees so that rest of the body remains still. If you manage to do that I'd very much like to know how!