Well, menopause is a possibility then I guess. I know that for several years before menopause my cycle had shortened (interestingly enough to about 24 days), and I had elongated, spotty periods. Menopause is just a thought...it might be that when you get your levo dose stabilized, you'll go back to your normal cycle.
You don't mention your age, but it sounds like perimenopause to me???? Your doctor is doing the right thing...starting you off on a low dose and increasing you slowly. What I would have given to have had your doctor!