2 periods in one month

I normally have a very regular menstrual cycle. This month i started my period as normal on the day that i was supposed to and it was very light and only lasted 3 days. 17 days later i started another period, but this one is heavy and has lasted 7 days. i can't be pregnant because my husband has been fixed. i know that it has only happened once, but i am still concerned because i have never had this before. Please let me know what could be going on. thanks. my mom started menopause at 35, I will be 43 in July. I also started thyroid medication 3 months ago for an underactive thyroid.