SI joint compression or instability in foot behind postures?

Hi folks

Is there any way of telling whether discomfort experienced in the sacroiliac joint is due to compression or instability without trotting off to get an x-ray?

After a period of practising the foot behind the head postures in Ashtanga, I’ve been feeling some discomfort in the SIJ and I haven’t been able get to the bottom of it. My best guess is that since all these postures call for extreme external rotation in the coxafemoral joint, and put us in extreme flexion, the likelihood of any discomfort emanating from the SIJ should be due to a compressive effect, since external rotation compresses the SIJ as does flexion.

In a posture such as yoganidrasana for example, we could practise such postures more safely by encouraging less compression in the SIJ by engaging the thigh adductors because this opposes the compressive effect that external rotation of the femurs may bring to the SIJ

right? or way off beam? Grateful for any input.

Thanks!
Y