I have sever lower abdominal pain that’s constant but also has sudden stabbing pain? Pain radiates to lower back too?

Some possibilities.. If one has very uncomfortable tummy pain, and is normally in good health, plus is not pregnant, the possibilities include: appendicitis (usually R lower abdomen), diverticulitis, kidney stone (very painful), constipation (crampy), stomach flu, etc... Most people with "moderate pain," more than mild pain, may need a doctor's evaluation to be sure it's not appendicitis, which would need surgery.
Go to ER. If the pain is severe and lasting for more than a few hours, it is best to be examined by your doctor. If this is from appendicitis, waiting too long may lead to rupture of the appendix causing life-threatening infections. However it is not possible to tell you what may be causing the pain but I am usually concerned when the pain radiates to the back. .