congratulations on the upcoming wedding; yours? Anyway, 5'2" and 128 pounds is a good healthy size. You have also done quite a bit to have a healthy lifestyle - that will help a lot in the long run.
You state you felt a mass in the right upper abdomen, but it wasn't picked up on US. It is very easy to feel the edge of the liver in that area in some people. The kidney is actually posterior (in the back), in the retroperitonial space. It is protected by the rib cage. It would be very difficult to palpate, even in very thin persons. Distended loops of bowel can sometimes be felt, but they come and go, as the gas passes through the segment. Flatulance can have many causes and be a very significant problem socially.
You should discuss your concerns with your physician. He/she can do a thorough exam, possibly repeat the US (or suggest a different set of tests), or even possibly set up a referral to a GI specialist. There are treatments for flatulance and since it is quite concerning to you, you should look into it further. Good luck.