You’re not alone, I mention this fact in this same comment thread. Decaf does have caffeine and if you’re sensitive to it then you’ll still develop sides. This isn’t from my experience (I quit completely at once) but from many experiences that I’ve read. It also happens if you take a bit too much dark chocolate. I’m not saying this is your case as this definitely isn’t the single universal source of anxiety and headaches but you may want to try quitting completely and see how you feel. I noticed it within 1-2 weeks but then I was taking 5-6 cups sometimes.