Hello thank you for your query. Back pain is common during early pregnancy. As you did not have regular periods, you might have been having polycystic ovaries or hypothyroidism, and both conditions go hand in hand with calcium and vitamin D deficiency. Early pregnancy is when the pelvis gets vascular and the uterine size starts increasing causing stretching of pelvic ligaments and muscles. Also, you tend to feel more tired and easily fatigued, and pain thresholds are lowered. Just to be on the safer side, consult your OB GYN about it, and ensure that it is not because of any impending miscarriage or uterine pain. That seems most unlikely since you do not have any bleeding / cramping / spotting etc. Take care.