I also had that at 20 weeks, it's just everything softening up and it will continue to be like that for a week or two. I went to the emergency room for it and all they said was to try to stay comfortable. Only super hot showers helped with the pain.
I have had lower back and hip pains from about 20 weeks I've had to go physio because of the ligaments loosening it pulled my hips out of place and just been told I have SPD aswell. Best thing is to ask your midwife. :) I'm now 28 wks 4 days