Hi, and thanks for your medical question on eHealth forum!
Tubal ligation failure is possible, but generally occurs in first year. After 2 years chances are very minimal. However take a pregnancy test and see. In all probability this is hormonal, due to either hypothyroidism or due to PCOS or due to pituitary suppression. Hence you need to test for these.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist once. Hope this helps. Take care!