This happens to me alllll the time. It is dissociation apparently, and i experience similar things to you like the sleepiness, heaviness, i get all warm and insular and talking takes a lot of effort. I seem to blink less and stare more too.
If i tell her the week after that i don't remember anything from the last session, she'll remind me of what happened, i kind of remember most of it when she tells me what happened but i couldn't recall that info myself.