A lot of factors, including stress, can cause changes in cycles. Depending on your age as well, cycle changes can occur with menopause and even early menopause with some women. It's very unlikely to be pregnant after tubal ligation, I can't say impossible, but still extremely unlikely. Changes in medications, diets, new workout routines can also affect cycles in some women.
For peace of mind, if you are late, take a test and you will know for sure.