I received a message from a woman who had some concerns related to her navel piercing:
I got my belly done yesterday but it looks quite bloated? Is it supposed to be? And also is it possible to loose weight whilst wearing a belly bar?
There could be some localized swelling but it would not be normal for a navel piercing to result in generalized abdominal bloating. In fact, I can't think of a single medical reason that a piercing could cause that to happen. More likley you have eaten something that doesn't agree with you.
Also, the ability to lose (or gain) weight relates to the intake of calories and output of energy. There is no relation between your navel piercing and your body's ability to gain, lose, or maintain your weight.
That said, during initial healing you need to be conscious of not injuring the area or causing excessive trauma during physical activities.
Below are a few related sections from The Piercing Bible:
Physical Activity and Exercise
Generally, you can exercise during healing if you avoid extremes of motion and prevent impact to the pierced area. Still, your activities often need to be modified—at least for a while—if they cause problems with your piercing.