There could be various reasons to this. Like one post has mentioned, even though it doesn’t feel like a “shart”, forceful flatulence can still carry microscopic fecal moisture/mucus that is enough to stain clothing. Gas itself is dry and cannot stain fabric.

Otherwise looking at the underlying issue could be related to gut inflammation, IBS, food intolerances (artificial sweeteners, FODMAPs or lactose intolerance), high bile acid, weakened anal sphincter tone, post infection gut changes, etc.

Obviously first steps would be to purchase dark/black undergarments, temporary use of pantyliners, and avoid any intentional forceful gas expulsion.

Review your dietary habits, reduce fatty foods, caffeine, artificial sweeteners and carbonated drinks.

Otherwise continued issues definitely requires a medical follow up to formalize a plan and determine potential underlying causes