Your symptoms sound identical to mine when I had gall stones. Pain radiating to the back, etc. I got many of the same tests you have gotten, too, to no avail.

Gas- try over the counter simethecone (spelling?), like Gas X. Try cutting way, way back on fat for a few days or a week to see if that helps. You could try reducing or eliminating these, too: Fried food, spicy food, carbonated drinks, caffeine drinks. You might get some relief, I hope.

Clothing- you have my sympathy. I'm older and retired now so I can do full-out comfy 99% of the time, but when I was working I couldn't wait to get home to get outta the clothes!
Try layering your clothes. A loose bra, under an opaque blouse with a sweater -- if allowed-- or a jacket- works pretty well. Dark colored, patterned blouses can hide a lot. Leggings- try the large or extra large to accommodate the waist bloat. You have to yank up the extra length of fabric under your skirt, but the leg itself should still be pretty clingy and not noticeable.
Wear an oversized shirt over pants or skirt that you close with an extender-- I used extenders early in my pregnancy so I could keep wearing some of my clothes a little longer. Then I used them during the gall stone time-- when I wasn't pregnant. Hey, if it works, why not?
Maybe break down and buy some basic stretch-top bottoms. If you wear a cute shirt or sweater over it, then no one will notice the fruggly-- but comfortable-- bottom.
My pants would fit fine in the morning when I left for work. By the end of the day, my stomach blew up 2 sizes and my waist band was killing me. I would take the extender to work and add it later in the day. A sturdy rubber band also works! HAHA. Binder clips make marvelous fabric holders, too! (I had forgotten about the office supplies as clothing ascessories. )

I know a woman who bought 2 bottoms for every 1 suit jacket. She bought the bottoms in 2 different sizes to accommodate bloating. Brilliant, if you have the money.

I hope you can get this all sorted out-- soon! (sorry about my spelling)