How old are you? My mom started going through menopause in her late 30's (as did her mom and her sisters). I know quite a few people are surprised that it can happen so early.
Otherwise, (and please take this with a grain of salt - I saw it on TLC's I Didn't Know I Was Pregnant - a guilty pleasure), there is a teeny tiny chance of getting pregnant, even with the woman having her tubes tied.
I think the only way to know what is going on, if you're still getting negative test results on HPT, is to go to the doctor. Cycles suddenly getting wonky can be indicators of other medical issues, too.
I am definitely not an expert, but I didn't want to leave you without any responses. Good luck!
Rae (27), DH (26), Lucas (6/3/12), #2 due in December