“Bloated” look alone can sometimes be misleading. Some common conditions with a similar appearance include being egg-bound, fat, having a tumour/cancer inside the body cavity and systemic bacterial infection leading to dropsy (where fluid accumulates in the body and their scales start protruding). Some diagnostic procedures your fish veterinarian might use to tell these apart include X-ray, ultrasound, palpation and fine needle aspirate of the abdomen for cytology examination.
If it is indeed egg-bound, you can try using Epsom salts in the water. For koi in outdoor ponds, you can wait till the next warm season, or use heaters to elevate the water temperature (gradually) to 22-24 degrees Celsius and maintain it for a period of at least 20 days. This will stimulate her eggs to mature. It’ll also help to have males in the pond to help stimulate release of eggs. Otherwise, by 20 days, she can be manually stripped. There is a hormonal induction method (e.g. injecting Ovaprim), but there might be complications if she’s injected too early and the conditions aren’t right. This is why such medicines are only available through a trained veterinarian. If it is a tumour, it may be possible to perform surgery to remove it. If it is a bacterial infection, antibiotics may be a solution if it is not too late.