Actually the pain makes complete sense to me. Like I said my uterus is tilted too, and I always have to go with the opposite when checking cervix position. For me when I should be SHOW at ovulation time is when it is EASIEST to find the opening to my cervix is it is SLOW...hehe. Anyway, I could see that the position of the uterus, as in when the cervix should be lower like right before the period that it would change the angle enough to put pressure on the sacrum and cause sciatic type pain.
Cyclic back pain can also be from your pelvis needing to be stabilized, which you can do through physical therapy. (this plug coming from a physical therapy assistant that used to have lowback pain and does not anymore from doing exercises

) What happens is the release of hormones during that part of your cycle makes the already unstable pelvis worse, allowing the SI joint to move when it shouldn't. Add to that the extra pressure from the uterus and voila back pain. Now, if you strengthen the pelvic musculature, it keeps the SI joint from being dysfunctional, which reduces pain.