I say it's fine, because I still read lying on my tummy. It has started to get a little uncomfy, but if your body is doing it then it can't be that bad. I tend to roll onto my back when asleep and figure my body is sensible enough not to do anything damaging whilst I'm not awake doing stupid things to it. Soon you won't be able to sleep on your tummy at all, so enjoy it while it lasts!