Hello, I Had A Baby 6 Months(July) Ago Via C

Hello, I had a baby 6 months(July) ago via c section and took thyroid medication during my pregnancy.
I bled for 3 weeks after the c section , than started my first period three months later. (Oct)
Than had another period in November.
Between the periods I had some dark brown, and sometimes black discharge. This occurred after the November period as well. I haven't had a period since November. So I have missed two.
Could this be because I stopped taking my thyroid medication (pure forgetfulness), or is something seriously wrong here?
My doctors were booked full all of December and still can't get me in til next week, and waiting is killing me.
I've also been fatigued and am very nauseous.
I know for a fact I'm not pregnant as I haven't been sexually active since pregnancy.
Please help.