Hi want to know, welcome and thanks for your question.  So sorry to hear about your breast cancer and now the declining bone density.  According to Medline (Nat Institute of Health) femara may cause or worsen osteoporosis.  Unfortunately some drugs that work well for cancer treatment, may cause other problems.  If the femara helped with your cancer then that's great, but unfortunately there can be a trade off with these meds when they cause other problems.  I hope the Fosamax works for you and increases your t-scores.  Below is a link from Medline that explains this phenomenom with femara.

http://www.nlm.nih.gov/medlineplus/druginfo/meds/a698004.html

Good luck with the fosamax!!  Be sure to get calcium, vitamin D and weight bearing exercise with the fosamax.