You had tubes tied 4 years back & had no problem in last 4 years. The failuer rate of Tubal ligation
,done by an experienced Doctor) is very less (0.7%)- so there should not be coseption.
Still in rare cases.over a period the tubes may get recanalised automatically &give rise to unexpected pregnancy
If you don't want the pregnancy at this stage, I suggest you to see doctor & get the thing clear.
Before that, you should go for blood pregnancy test . to
exclude/confirm pregnancy.( Blood test
is +ve by 8 th day of ovulation.,even
before periods )