I get this if I lie in certain positions and sometimes when bending down for a while (such as hoovering).
I have arthritis and my rhumatologist picked up on something on some spinal xrays they did and said there was something abnormal on my sacroilliac joints (lighter than they should be).
Went for an MRI so she could have a closer look (they lady scanning me had to help me up off the table as my back locked in that haha) but she seemed happy with the results so not sure what is causing it.
I've always found osteopathy really helpful as they seem to be able to release a lot of the tightness and suggest exercises. It tends to go back to how it was but has been reducing it over time so it might be worth a try.
Definitely get it checked out with your GP though in case there is anything specific causing it.