Quite bad pain in pelvis and back

This is my second pregnancy, and I’ve never felt this pain before. It’s not round ligament pain ( I remember what this feels like) or pelvic girdle pain( it’s happening when I’m not moving). I am getting quite sever pain that starts low in my pelvis and moves around into my back, it lasts for a minute maybe 2 and eases with deep breathing and leaning backwards / loosening clothes. It’s happening frequently this week and is quite painful, not just like a small twinge. Has anyone experienced this? Worrying me as I’m pretty sure it’s not ligament pain. Thanks x