Well, once in a blue moon, a tubal can fail. It's not common and happens because either the clamps came loose if you had the procedure done that way or scar tissue formed in such a way that the tube is 'sort of' fused together so an egg can end up fertilized. BUT, when pregnancy does occur after a tubal ligation, it is most often a tubal pregnancy, or ectopic. That is not a viable pregnancy and can be fatal to the mother.
I would guess though that you are not pregnant. Hormones can still get out of whack and periods can be late. Causes are weight gain or loss, illness, changes in routine, stress or just the wind blowing the wrong way. It happens to everyone. And you had negative tests.
If you still feel unsure, then get a blood test at your doctor's office. Not a bad idea if you don't get your period in a week or two just to make sure as again, an ectopic pregnancy is dangerous. good luck