PLEASE ANSWER! Belly button pain but only when I move/stretch. If I stay still I'm fine. I'm on period. Is it related? Should I be VERY worried?

Not urgent. Mid-lower abdominal pain with pelvic movement suggests symphysis pubis, SP, subluxation with SP ligament strain/pain. Menses-associated Relaxin hormone engenders loose pelvic ligaments; more likely to manifest in those with Hypermobility Syndrome, HS, or if loose sacroiliac joint & concommitent low back pain. Panic attacks oft occur with HS & is known relationship HS & evolution of fibromyalgia. .
Likely muscle strain. given it is only there with movement or activity. Unlikely to be related to your period. No need to worry.