The rhomboid muscle can be responsible for pain at the side of your shoulder blade.
Mostly desk workers suffer from trouble in this muscle as they spend lots of time working in a round shouldered position.
This constantly stretches the muscle and will eventually overload it.
When your rhomboids contain trigger points, you might feel pain at the inner side of your shoulder blade and between them, respectively. That is the most common painzone for those muscles.
The darker the red in the picture, the more common it is to feel an ache in the respective area if your rhomboid muscles contain trigger points.
Usually this muscle will give you pain at rest when it contains trigger or tender points.
Pain may be felt when you lye down on the affected side. But also tasks that involve reaching out for something like a cup from the shelf may be painful.
In the latter case the muscle gets stressed via a stretch/elongation.