Pentagon - as far as i know (I'm not a HCP) sleeping on your back isn't bad in itself. For some women, it can cause real problems, as the baby presses on the vena cava (or one of the major blood vessels) which can cause the mother to get dizzy, feel sick or even pass out. If you're not getting these symptoms on your back then I don't believe there is any harm to it, so I really wouldn't worry.
I don't know where this thing about lying on your left (or is it right? I'm lexdysic so can't tell the differenc) came from.
If you haven't felt the baby move, the usual advice is a cold sugary drink and having a lie down (sounds bliss anyway!).
Also (and time to get on my soap box) - ladies, please do make use of your hospital's day assessment units if your baby's movements slow down or stop. I can't remember when they'll start accepting you, but I went with DC1 and they were so amazingly lovely, and supportive. DC1 didn't stop moving totally, but at about 31 weeks the movements slowed right down for more than 12 hours, so I went in and they were fab. They were so pleased with me for coming in and never felt like I was wasting their time. DC1 was fine (though it took an overnight stay for me to be released) but they said they probably "saved" about one baby a week through timely intervention. I don't want that to scare you, but modern healthcare and technology is amazing, so use it!!