Chick start doesn’t have enough b vitamins for waterfowl which may have suppressed her growth and immune system early on.
What brand of game feed is she on?
Also what does her droppings look like?
Are you able to weigh her?
The popping in her gut is gas, which is usually caused from a gut infection but could also be due to diet. Something like Giardia, coccidia, Candida, clostridium, salmonella, overpopulation of e.coli, or even worms are some of the causes. I have one that has a recurring issue with Giardia and enteritis “clostridium“ and his gut sounds like that when he’s coming down with it again.
A gram stain and fecal float could give some ideas about what may be going on but if a vet isn’t an option or you don’t have one in your area that will see waterfowl “which is common” you’ll need to treat based on some educated guesswork.
If you can find liquid B complex and a liquid multivitamin I would start giving it to her asap. It‘s best if you administer it down her throat with a syringe “6ml or 12 ml/ 12 ml is safer.”
Some other things you can try is giving her safeguard horse paste or goat wormer. Fenbendazole will kill worms but it is also effective against Giardia.
Metronidazole can be bought as fish antibiotics, it’s effective against Giardia and clostridium.
Tylosin is effective against clostridium and a number of other nasty stomach bugs. It can be purchased in a powder form for pigeons.