I'd like to add that I have several T score changes of 0.2 (up or down) at different bone sites in my 3 annual DXA reports (same machine same tech). If they are clinically significant, an asterisk (*) was indicated right next to the bmd %change. It's interesting that one 3.6% change (with Tscore change of 0.2) in bmd is "significant with 95% confidence" while the other 3.6% bmd change is not. A closer look showed they are for different skeletal sites and LSCs are different.

Your original post of T score change from -2.9 to -3.5 appeared to be large but you had corrected later as -2.9 to -3.1. I definitely hope it is the latter. Double check to see if 4% change in bmd is signifcant if you have LSC info. There are talks about the common mistakes when scanning Lspine. One of which is scanning L2-L5 by mistake instead of L1-L4 according to books and online information. If this is what happened then a rescan could be warranted.

Checking bone turnover markers as @gently pointed out is one way to monitor if an anabolic such as Forteo is working well in early months of treatment. At 6mo of therapy, it is a good time to do a lab. Medical care in a rural area could be a challenge. It won't hurt to communicate with your doctor though. Some of the members on this forum use P1NP and CTX. There are threads discussing pros vs cons of bone markers and steps to take to ensure a more accurate reading. In case the managing doctor is not into bone markers, we as patients could order the lab online ourselves (by paying out of pocket 🙁 ). There is a thread for this info with ordering codes and costs. The drawback in your case might be that you might not have a baseline (me either) prior to Forteo therapy. The variability in bone turnover markers due to premetrologic and metrological causes could also make the results hard to interpret.