osteoporosis and renal calculi

I have had renal calculi for over 14 years. I do not have hyperparathyroidism or other diseases known to cause this problem. I also have osteoporosis. I am concerned about the low calcium diet since I am still far above norrnal levels for urine secretion of calcium in 24 hr. urine. (500). I am continuing HRT (premarin and progesterone), but I have not taken any Fosamax for about 1 yr. due to gastric reflux.
I have two questions:
1. What causes me to pour so much calcium into my kidneys?
2. Why is Fosamax only giving me a slight increase in bone density after over one year of taking the medication, and why did I still pour out calcium when I Fosamax?