Hi, Thanks for your question. Pleas rate my answer if you are satisfied. Lower back lifts most of your body weight. There are a number of possibilities about why are you having that pain. Looking at the triggers of your pain, it seems you have that pain when you put strain on your lower back. It suggests that you might have torn a ligament or muscle during an activity like lifting something heavy or straining over your back too much, without realizing it.There could be other reasons too like, you may have osteoporosis (very weak bones due to vitamin D or calcium deficiency) or even more serious conditions like spine problems. In any case, I suggest you see an orthopedics specialist and get an X-ray done.