Change in period

I have been try for about 6 months now, and my periods were fairly normal and consistent, for the last 3 months they are different, shorter, lighter, and tmi, but a differnt color. I am making a doctors apt today, but I was wondering if anyone had experienced anything similar? I do have hypothyroidism but I take meds and my levels have been normal, I have also taken multiple pregnancy tests and all have come back negative. Thank you for any help/ advise