Lower back pain, weak legs and arms. does this mean i'm having a growth spurt/getting taller? 16 year old girl, got first period 15 yr 0 mo

Not a growth spurt: you should see your doctor. this could be a sign of disc disease, guillain-barre, or spinal cord impingement. Most growth occurs before girls get their period. After menarche girls have modest change in height.

Not related: By 16 most girls have had their growth spurt which is usually between 12-15 yrs. weakness in arms and kegs can be from a lot if different other reasons like low iron and other minerals in body like calcium ets.