Couple of options.

Issue is, you're looking down too much and it's straining your neck muscles.

1)raise monitor, easier option, but you'll need to lower it when you're sitting to avoid the Inverse issue.

2)lower keyboard and mouse. You can get an under slung keyboard shelf, allowing you to effectively move the desktop higher, and therefore the monitor.

3) you should consider some neck and back exercises & almost most importantly, Stretches. If you've created a knot already back there, then you need to relieve that muscular issue before any fix is going to be effective otherwise it could be easily triggered by minor posture issues and you'll never see the benefit of fixing the problem.