I recommend that you talk to your OB GYN doctor
or your primary care doctor
about this issue. It is a little hard for me to provide much of an answer to your questions without having more details. For example, I am not sure if you had the tubal ligation a long time ago or whether this was a recent procedure.
Definitely there can still be a small risk of pregnancy after a tubal ligation. It sounds like you have already done a pregnancy test, but you may wish to have your doctor perform an examination and another pregnancy test just to rule out this possibility.
If pregnancy is ruled out, another possibility is that this is simply related to the tubal ligation procedure. In one study of women with tubal ligations versus those without, the ones who had undergone the procedure were more likely to report irregularities in their menstrual cycles.
Finally, it is possible that your periods are spacing out due to aging of your ovaries (again I am not sure how old you are). As women age, their periods due tend to get more irregular, either shorter or longer, and this can be a normal part of the aging process.
Please talk to your doctor for more information.