Definitely ask for a blood test. I have hypothyroidism which was causing my periods to be more frequent and heavier. Having scanty or infrequent periods may mean that your synthroid dose is a bit too high. You really have to push drs sometimes. I complained for 2 years (starting when my 4yo DD was 2) about my periods and tiredness, but my TSH was still "normal" while on 50mcg synthroid. After my m/c last spring I did a lot of research and had a heart-to-heart talk with my dr who ended up consulting an endocrinologist. The endo said that while on synthroid, you should try to keep your TSH closer to 1.0--mine was at 3.11. Sometimes I think that people with thyroid issues know more about it than the doctors do....
Best wishes.
If it were me, I would request blood work. That will tell you for sure whether its a pregnancy or not. It sounds unlikely with your tubes being tied, but its certainly not impossible. If in fact they rule that out, find a Dr. that takes your concerns more seriously. I completely believe in women's instincts, and we need to take control of our medical situations. We know our bodies better then any Dr. does, and remember, he/she works for you. I wish you all the best.