Beginner struggling to sit with straight legs/straight back
Hey everyone, I just started paying attention to my flexibility and I'm trying to understand what's going on with my back/hips. Attached some side-view photos while sitting on the floor.
My main question: why can't I sit on the floor with straight legs and a straight back? As you can see in the photos, my lower back rounds pretty heavily instead of staying neutral.
The hardest position for me by far is downward dog, I can really feel that my back isn't straight there either.
Some context: I'm just starting out, so this is all very new to me. In some stretches I feel more of a pinching sensation than a stretch/pull, mostly around the hip/pelvis area. Not painful, just different from what I'd expect a normal stretch to feel like.
Any insight on what's limiting me here (hamstrings, hips, pelvic tilt, etc.) and where I should start would be really appreciated!