If you had your tubes tied, it is very doubtful that you are pregnant (you did have a follow-up fertility test to confirm it worked, right?).

There are a lot of things that can cause back and stomach pain, including gallbladder issues, kidney stones, intestinal problems, etc. You should go to the doctor and have it checked out.

If the pain becomes severe, especially if you have other symptoms like nausea, you should go to the hospital, because it could be appendicitis.