Your cycles have ranged from 31 to 43 days recently, so going a bit past your usual length isn't unusual, especially while you're on
thyroid medicine. The back pain, breast tenderness and bloating you're describing are typical signs your period is on its way, not signs of anything going wrong. Thyroid levels that drift out of control can also push ovulation and your period later, so that's worth a look too.
Next Steps
Give it another 7 to 10 days past your longest recent cycle before calling this a true delay. If your period still hasn't come by then, get your
TSH checked again since it affects cycle timing, and see your gynaecologist to review the overall pattern.
Health Tips
See your gynaecologist sooner if you get heavy bleeding, severe pain, or your periods stop for more than 2 cycles in a row.