Hi Jo: This is a tough question and almost impossible to answer without more info. You didn't mention if you have gone through meno, or if you are peri-menopausal or pre-meno.
So many different things can cause bone loss and I don't know if you have any of them. For instance, have you taken prednisone, or epilepsy drugs, or do you have any secondary cause of osteoporosis i.e. celiac, parathyroid problem, eating disorder etc., all of which can cause early bone loss. Does this study include looking for secondary causes of osteo for someone your age and meno status? That might help you more to answer this question and tell you why your score is in the osteoporosis range at your age, however approximately 30% of pre-meno women have osteopenia for one reason or another.
Generally estrogen, produced by the ovaries, helps to keep bone mineral density stable. Usually your bmd changes very little until you become peri-menopausal or post-meno. If you have gone through surgical meno, or some other form of early meno you may see a loss, and if you haven't then I suppose it's possible to see an improvement in that short amount of time.
Hopefully calcium and D will help your score by either maintaining it, or improving it, but I don't know if you will see this improvement in 2 months, but let's hope so.
Good luck with the study, and I would ask the Dr handling this, all these questions since they should be able to tell you a lot more, knowing your medical history.