sleeping on back

Li

so boyfriend has been staying over for about 2 weeks now and is staying till baby is here. He says i constantly go on my back and will stay there for hours and when he moves me i just go back there. I’m worried as i can’t stop myself and don’t know when i do this.. i do put myself on my side to sleep but don’t seem to stay there, before i was pregnant i was always sleeping on my belly and back and never my sides. I’m 38 weeks pregnant can anyone say if they have slept on their back sometimes and baby has been ok? Baby is still moving but worried as he used to move in the morning but now only moves in the night or afternoon (i’ve told my mw but she says it’s most likely because he is engaged)