What Causes Headaches, Dizziness, Tiredness And Fatigue?

I started dr. Bernstein 3 weeks ago to get me motivated. I have done it in the past with no problems. I had a baby back in sept. I was recently diagnosed with hypothyroidism. My level was 26.18. I started eltroxin 2 days ago. I am so severly tired, dizzy,headache and no energy at all. Could it be the pills