Hi, Welcome to HCM. Irregularities in the period by 2 to 4 days do occur and it is fine. Chances of you being pregnant is very slim. But i advice you to repeat the test at least a week after. Tubal ligation is not a cent percent preventive method for pregnancy. Though comparing other methods of contraception, tubal ligation has minimum failure rate. A woman can become pregnant any time after she has undergone permanent birth control method such as tubal ligation. This is due to the margin of error which every procedure has. Failure in rare cases occurs because the fallopian tube which is ligated grows back together. The egg comes down in the tube to be fertilized and then moves in the uterus. But the chances of such cases are very rare. Accuracy of urine pregnancy test kit is nearly 98%. But there high chances of it being negative when done within 7 days of missed period. I advice you to repeat the test after at least a week after missed period, preferably using early morning urine sample. If after week also it is negative and no period i advice you to see OBG specialist for examination. Hope I have provided the information you needed. Wish you good health. Take care.