there could be a number of things causing this. sometimes women just have really light periods it's normal for this to happen occasionally. light brown blood just means it's been held in the body longer and so isn't as fresh. it could be perimenopause that isn't uncommon and can happen for a few years before true menopause. it could be caused by stress hormonal shifts or any number of other factors. i think a pregnancy is pretty unlikely with the tube-tying you described.
i am a 44yr old female. had my tubes tied burned and clipped. but in january my period was very light 4 days light brown