Low back pain like im on my menstrual period? What could it be? Do I have pinched nerve? Is it fatal?

Sciatica, Not Fatal. Pain in the distribution as you suggested describes the pattern of the nerve that travels in the leg called the sciatic nerve. The sciatic nerve is not the cause of the pain source, it is the result of an irritated nerve typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease requiring further evaluation by a spine specialist.
multiple reasons. Pinched nerve, compressed vertebra, ruptured ovarian cyst, kidney stone, gallstones, diverticulosis, etc. There are so many possibilities that you need to be seen by your primary care doctor to begin to sort out what it might be. This sooner you seek medical attention, the sooner you will get your answer and won't have to worry about it being fatal.