Why is my stomach flat when lying down, but "swollen" and bulbous up to 3" above by incision when standing up?

The abdominal wall is broken down into multiple compartments. The most superficial compartment is the skin and the superficial fat. Deeper to this is Scarpa's fascia and deep to this is the deep layer of fat. At the bottom layer of the abdominal wall is the rectus fascia.  
Traditional tummy tuck operations remove all of the above mentioned tissue down to the level of the rectus fascia. This necessitates the use of drains to remove the serous fluid which can form as a response to the intervention of the surgery.
 By performing several maneuvers at the time of the operation the likelihood of serous fluid accumulation at the operative site can be diminished. These include but are not limited to progressive tension sutures and Scarpa Sparing maneuvers to keep in place the bodies lymphatic system 
 For persistent swelling in the postoperative period that appears to change with positions consideration for retained serous fluid can be investigated. These are important discussions to have with your plastic surgeon. 

Thanks so much for reaching out. You are still relatively early in your postoperative phase. This can be due to a few reasons, the first being swelling. Swelling can last up to six months after tummy tucks even 12 months. And the simple force of gravity keeps it so the swelling occurs at the lowest point of the surgical site.   You can also have a hernia, however this is very unlikely. Also sometimes just the dynamics of the way your hips are to your spine have it so that your tummy will always be a little bulbous lower down. I hope this helps :-) 

Sounds like swelling by your description which is very common at 7 weeks. In order to be sure, you would need to be seen in person, but it does sound like swelling.