I was experiencing this last night. My left shoulder was spasming and I started getting a weird little "twinge" in my scalp on the left side. Then it progressed to a larger twinge, and finally felt exactly like an ant was crawling on my head! I've had this before on the other shoulder. This article says it's odd, curious and rare but I know two other people who have this! My mom has it from her fibro, and my former massage therapist has it from a herniated disk in his back.