Feeling stressed can cause a little of muscle tightness, but simply relaxing won't necessarily bring tight muscles back to a comfortable state, otherwise everyone would wake up in the morning feeling supple and pain free. Feeling stressed also makes us more aware of muscle restrictions and pain, possibly because it's yet another problem on top of all the other problems we have, and we are often concerned about what is causing this pain . When we are stressed we are more likely to default to comfy postural positions, even if this position is leading to the pain pattern. Changing to a different default posture seems too taxing, and we don't have time to try them. When your a bit less stressed, it's good to try to make a good ergonomic set up for your activities, including always working on a computer at a desk, with wrist support, an eye level monitor, and a good chair.
The principle of Alexander Technique is that habits feels right and comfortable, while new habits feel tiring and difficult. It takes time to develop good habits because we are naturally reluctant to change.