I was also diagnosed with osteoporosis at a young age - 28.
I had not been taking any steroid medications and my parathyroid functions came back normal. Perplexed, my doctors blamed the osteoporosis on my Depo-Provera injections. I quit the Depo shots and began taking Fosamax 70mg once weekly. After two years, there was very little improvement in my bone density.
Several weeks ago, I finally went to see an endocrinologist and asked to be evaluated for Cushing's Syndrome. I felt that I was experiencing many of the symptoms associated with this condition, one of which is bone loss. Surprise - it turns out that I do, in fact, have Cushing's Syndrome. In my case, it's caused by a small, benign tumor on my pituitary gland. I'm in the process now of choosing a neurosurgeon to remove it.
Nobody thought to test me for it before, because Cushing's is quite rare. In fact, my endocrinologist even raised an eyebrow at me when I told her that I thought I might have it. The funny thing is, I would never have learned of this condition if I had not been searching the internet one day and accidentally stumbled upon some information about it.
Just thought I'd share my experience with you. If your doctor can find no other cause for your osteoporosis, you might want to consider asking him/her to evaluate you for Cushing's Syndrome. Best of luck.