I think I've figured out something after 25 years of this. I know they are (or can be) related to how stressed out I am. But for the first time ever, I noticed something in bed this morning (because I was awake). I woke up (had had them), changed, sheets etc, turned up the heat, turned on electric blanket, went back to bed. Had shooting pain from my stomach, you know the kind that permeates like a ripple through your body? It was a super cold/ painful sensation, and it made me feel freezing. Then I 'bundled up' with the covers, each piercing one felt cold, and made me feel cold, but I started to sweat. And the pain is related (started) with stomach/ bleeding issues, so I guess in that way stress-related (as well). So maybe it's the experience of physical pain (as we sleep)?
And you know, it can be a cycle: stress causes pain, but pain causes stress, etc.