I don't know the technical reason why this happens but I do know it happens to people like us. I have found when I'm overtired, this happens more. It also happens more when I'm not properly hydrated. I'm dealing with it right now. I was progressing so well, then found out I'm pg. Kinda not in the recovery plan. So now I feel like I'm taking a couple/few steps back. :( It does get better as you get better. I went months and months without this happening at all.
Hugs and hoping you can sleep well. I know how not fun it can be.