mine normally does too. i do get fizzy limbs after about 5 mins of laying like it. gives me a bad back too. its actually impossible not to lay on your back, lounging in a chair, in the bath, waking up on your back, ultrasounds and heartbeat monitoring, even when in labour you see woman on their backs for hours on end. I dont get why the profs say dont do it because most of the time we are on our backs at apts anyways :S i always sit slouched as i feel like im squashing him if i was upright. i dont think it can harm LO that much. But try to stay flat off your back, if your laying partially up and not flat then you should be fine.