Sounds like me when I don't take my ADHD meds, you might consider speaking to a healthcare professional if it's affecting your life to this degree.

It's important to move around every now and then, making sure to stand up and stretch your back and neck muscles goes a long way to preventing stiffness and sore muscles.

As someone who's been in and out of physical therapy for neck-related reasons in the past, it's especially important to remember that sitting in any one position for too long isn't good for your body in the long term.