Pelvic pain when lying down

As the title says, I have pelvic pain on my left side, but it really only flares when I’m lying down. Specifically, it’s when I lie on my sides. If I roll onto my back, the pain slowly goes away.

The pain is a constant, dull ache rather than sharp jolts of pain, but I’m honestly not sure what it could be. It goes away when I get out of bed too.

This has been occurring for maybe a couple of years, but it comes and goes. Happens maybe every 2-3 nights.