If so, you're practicing poor posture, which is causing your neck and shoulder muscles to tighten up. Remaining in a poor posture for extended periods puts a strain on already tense muscles.
Fatigue sets in as blood flow decreases, resulting in knots and muscle spasms.
This causes even tighter muscles and more severe symptoms, which cause tension headaches. Unless you take action, they will plague you more often and become progressively worse. Sit up straight and stand up straight. If you sit all day, get a chair with good back support. Bring your work surface closer to you.