If i bend down i can't straighten my back and suddnly my shoulder's are hunched forword, i used to sit straight.. why is that?

Doctor can check: Such symptoms are hard to describe in a few words. A primary care doctor, an orthopedic doctor, or a physical medicine (physiatrist) doctor has to see the patient move in and out of the various positions to see where the "locking" or the "tightness" is. The causes may be bone problems, muscle/tendon/ligament problems, or even nerve problems. Sometimes, xrays are needed. Physical therapy may help.

Lumbar strain: The most likely answer is lumbar sprain / strain.