Everyone here is absolutely right about the baby's position. The baby doesn't even get into birthing position until the very end of the pregnancy, so certainly at 18 weeks you don't have to worry if he's upside down, right side up, facing left, facing right, etc.
As for the food thing, laying on your tummy is not a good thing, and the further along you get, you should concentrate to always lay down on your left side because every other position has the potential to cut off some of the oxygen or blood flow to your uterus. Not to mention that laying down right after eating will increase your chances of heartburn/reflux. A woman's body is designed to provide maximum nourishment to the fetus---no special tricks are needed. In fact, if you want to do anything the best bet would be to take a nice walk after eating rather than lying down---it will stimulate your circulation, providing increased flow of oxygen, nourishment, etc., to the placenta, and is in general just a really healthy after-dinner habit. : )