How effective is a tubal ligation? Could I be pregnant?

I had a c-section 4 years ago, right after the c-section I had my tubes tied, my period is 4 days late, I am nauseous, I am constipated, I am dizzy, my head has been hurting, my lower back hurts, I have been peeing more, and I have a white l discharge similar to what I had early in my other 3 pregnancies. How effective is a tubal ligation? Could I be pregnant?