My baby has been transverse off and on the past couple of weeks - it is so obvious when he is because like you I can feel movements at either side near my hips, I can feel his back sideways across my belly, and my uterus is noticably "empty" up at the top (fundal height even seems lower)...but it doesn't hurt at all. It could be that you are just sensitive in the areas your baby is pushing harder against, or your pelvis is more narrow and therefore it's not comfortable when your baby is transverse...you might have been feeling significantly more movement just because in that position you can feel the baby more than you can when it's vertical. I read other women say that when their baby moves it makes them feel nauseated - could just be the feeling you get when the baby puts pressure on certain organs.
As far as the placenta previa...I haven't heard that, but it does make a bit of sense if the placenta is at the bottom of the uterus, it may be taking up some room down there and the baby may want to go the other direction to stretch out more. Don't know about that.
Overall I'm sure everything is fine (it's likely the baby was wiggling into a comfy position, got there, and went to sleep for a while), but trust your intuition and if you feel you need to make a call I would go ahead and do that.