? That is a common area in the lower back that can have subluxation, causing pain, and also decreasing flexibility.... There are also ligaments in this area as well as overlying muscle which can spasm (and cause pain).
Due to the SI Joint...
Go see your family doctor! S/he is trained to evaluate the cause of your pain and treat it. You can probably safely ignore occasional pain but if it persists and affects your sleep, it's time to get checked
This community covers anxiety disorders and other mental health issues, Sophie.
The posters at Wellsphere's General Medicine community should be able to provide a more authoritative answer.