Hi fifi,
First of all, what chelator are you using? L1/Defriprone/Kelfar is known for causing backpain, I too suffered sometimes in past when I was taking it.
Next, have you got your Vitamin D and calcium levels checked? Low Vitamin D levels often results in mild to severe pain in back/spine and legs, also causing Osteoporosis. I'm an Osteoporosis sufferer, in 2009 I was also hit with low Vitamin D levels and thereby causing Osteoporosis. However, today I'm better but still struggling to gain my lost strength and cope up other issues.
So I'd advise you to get your Vitamin D/D3 levels as well as serum calcium checked. Depending on your results, if Vitamin D maybe very low, you might be prescribed a small injection, which'd easily raise your D levels back to normal. Also, you're required to take Vitamin D daily, and go or stand into Sunlight, as it's very good and natural source of Vitamin D, if available in your country (for example, you might be living in a snowy country and thereby unable to get most of it from the sun).
-P.