Stomach or abdomen? There's a difference.
The stomach, both the new pouch and the old stomach are definitely swollen for 2-3 months after surgery due to the trauma.
If it's your abdomen, then there are several factors: surgical trauma (your body sends fluids to protect the "damaged" area. One of the reasons many surgeons put in a drain is to provide an outlet for this excess fluid. However, even after it is removed, you can still collect fluid there. Your body will eventually reabsorb it, but surgeons will ofter aspirate it in the office with a long needle. This usually does not hurt at all as you remain numb for quite some time after surgery. And if you had lap RNY, the cause could also be the gas they use. The gas can take a 2-3 weeks to fully dissipate and it can hurt like the devil. I had open RNY with no swelling at all in the abdomen, although my pouch/stomach as surely swollen, but had a lap hysterectomy last year that and the gas was by far the worst part of the recovery.