She needs vitamin C. You have three options.
One, get some Oxbow Vitamin C from a pet store. This is what you're looking for, but don't wait to order it online from here: (broken link removed)
Or two, get some Oasis Vita Drops Vitamin C for guinea pigs. This is the stuff, and it's available from Petco:
https://www.petco.com/shop/en/petco...centrated-high-potency-pure-c-for-guinea-pigs
Or three, go to a pharmacy and get some plain vitamin C tablets. Children's chewables are good if you can find them, because the pig will often eat them by themselves. But an adult tablet will do. See if you can find one that's 100 mg of vitamin C or less.
Give your pig 100 mg of vitamin C daily for a week. You should see a drastic improvement in that amount of time. At the end of a week, drop the dosage back to about 35 mg per day.
You need to change her diet so that she gets enough vitamin C from vegetables. Her pellets should have stabilized vitamin C in them. And bell peppers are the best vegetable source of vitamin C.
What exactly are you feeding her? What kind of hay? And how much does she get? What kind of pellets, and how many does she get? What kind of vegetables, and how many of each.
For the gas, if you've got a vibrating pillow, put her on that for about 20 minutes at a time several times a day. Or you can use an electric toothbrush under a towel, or stand beside her and hold her while you put her on the top of a running clothes dryer. Infant simethicone (available at any pharmacy) would be better than Mylicon.
Let us know what you decide to do, and how she gets along.