What Causes Light Period And Negative HPT While Having Hypothyroid?

I am concerned that I might be pregnant. I'm on the birth control pill Giavani, and have been for two months. Previously, I was on Yaz. There was no gap in usage. I last had sex in mid July, on about the fifteenth pill in the pack. I had a heavy period two weeks later, on schedule. This month, my period was still on schedule but very light. I took a pregnancy test two weeks ago, and it came up negative. I have Hypothyroid disorder, which is being treated with medication.
I have been very stressed out thinking about this possibility, because I'm a year from my college graduation. Should I still be concerned that I might be pregnant.