There are numerous causes for bleeding between periods.
Hormonal contraceptives, especially if you’ve recently switched to a new brand can cause spotting in between periods. It take weeks to months for your body to adapt to the new level of hormones.
Your history of hypothyroidism can also be contributing to your spotting especially if you’ve been inconsistent with your medications. Hypothyroidism (an underactive thyroid gland) can increasing the frequency of periods, cause heavy bleeding, and even increase the likelihood of spotting between periods.
Ultimately, it requires a detailed history and a physical exam to determine the exact cause of bleeding. Any bleeding in between periods, especially for patients your age, is abnormal and could be a sign of cervical or uterine malignancy. Therefore, you should visit your gynecologist to be evaluated so (s)he can determine the cause of your spotting.