You are doing great! Congrats on 3 weeks!
We all have the stories of what our bodies either had or currently is putting us through as we enter sobriety. Headaches, shakes, anxiety, depression, upset stomach, nightmare (this one is mine), and the list goes on and on. Your body is trying to force you to use and/or making it hell if you don't. For me, it was 10 years of drinking that my body and mind became accustomed to. It was pretty pissed off 101 days ago when I told it that it had been a long run, but I was done now. Give yourself time. It too will pass!