Being sick, (like fighting an infection/UTI) can cause periods to be late. You are probably not pregnant as you had sterilization done, right? That's not to say pregnancy never happens after a tubal. The clips can come undone or if the tubes were severed, they can form scar tissue that reforms tubes. The incidence of pregnancy isn't high but of those, they are very often tubal pregnancies. Which I'm sure you know are not viable and can be life threatening to the mother. I would take a pregnancy test but expect it to be negative. good luck