Blood between periods. With my partner.

I have been facing blood between my periods. In short, two times I am bleeding in a month with the gap of 12-14 days. This is not the first time, second time it is. For the very first time (i.e last month) when i saw brown thick blood I got worried amd therefore consulted dr. In gurgaon. She said that my last periods were nt enough so this is my old blood and concluded hormonal imbalance. Provided me with 3 days medicine. I took those piils for 2 days and after 3 days bleeding stooped. P.S- I have thyroid( H/O) since long time and I take altroxine 100 mg since 17-18 years now. My periods are always on time, never faced any delay or problem despite of thyroid but this bleeding two times in a month makes me worry. I am 25yrs old. Also, I had protected sex with my partner.