Hi Rinat,
Lecithin seems very beneficial for you. Just know that It is a lipophilic substance, so it can accumulate in the body. When you begin to feel this pain in your back, I suggest you take a break and stop taking lecithin until the pain disappears. You can then begin to take some lecithin again, maybe at a lower dose, and stop again whenever you have that back pain, which may be a sign that your body currently have enough in reserve. You can stop supplementing lecithin for a time without losing the benefits, because you have lecithin accumulated in your body fats, which will be liberated when you stop taking more of it through supplements. When the pain disappears, that means you have used up the lecithin in excess in your body, so you can slowly resume to take some of it.