Since you are so underweight, you need a plan to take in many more calories in a healthy and low carbohydrate manner. I suggest that you eat plenty of nuts and seeds. These are high protein, low carbohydrate(and the carbs present are almost all complex), and high calorie. Some people can tolerate peanuts, while others can't. Sunflower seeds are good, as well as all the other nuts(one shouldn't eat many Brazil nuts though.) I stayed away from nuts most of the time when I was overweight, but eat plenty of them now. Nuts are a good source of magnesium, zinc, and other minerals.

Your back pain might be due at least in part to a deficiency of vitamin D. Many people have a vitamin D deficiency. Have you ever had your vitamin D levels checked? If not then it is probably safe to take at most 2,000 units a day without having them checked(or perhaps just 1,000-1,500 due to your low weight?). Vitamin D is very important for the immune system and for proper calcium utilization. Are you taking vitamin A supplements? If not, then take 10,000 units a day A palmatate(not beta carotene). Have you tried eating raw garlic? A clove(section) a day after a large meal cut into small pieces, chewed well and swallowed with plenty of water is a good way to start.

Pehaps after you stop the chelation you could do a parasite cleanse if you haven't done one for at least several months. Many people feel better after doing a parasite cleanse. Vitamin stores sell parasite cleanse formulas, most of which have wormwood, black walnut green hull extract, cloves, and perhaps other herbs as well. Some see evidence of parasites while doing a cleanse, while others just feel better later(many parasites are microscopic). The parasite cleanse herbs also fight candida.