On my first period after breastfeeding, and it's now lasted 2 weeks and is still heavy. I have thyroid issues, too. Is this normal?

Maybe. First menses after discontinuing breastfeeding is often heavier than usual, however make sure your thyroid condition is under control, as this could also cause heavy long lasting menstrual bleeding.
See your doctor. Thyroid issues can flare in the months after a pregnancy, and can cause changes in periods. Lab tests can tell if your thyroid is causing this problem.