I just wanted to give you a few tips regarding Minnies fresh food diets,
If you've been feeding her iceberg lettuce, you should stop right away. It's far too high in water content and can cause stomach problems. Try red leaf or green leaf as a safe alternative
Secondly, earlier on in the thread you mentioned that she liked cabbage, which unfortunately shouldn't be fed very often at all. It's a gassy vegetable (along with sprouts, broccoli etc) and can cause guinea pig bloat, which can be quite serious. You mentioned she's developed a stomach, and if you've been feeding her gassy vegetables that may be bloat. If you tap on it, does it feel hollow? If so, you can give her half a childs dose of infant simethicone, and if you have an electric toothbrush (or anything else that gently vibrates like a phone or neck massager etc) put it under a pillow with her on top of the pillow, that should relieve the gas a little.
I hope you don't think we're criticising your ownership abilities, you clearly love Minnie very much (One of my girls is named Minnie too, by the way!) and have obviously just been led astray regarding diet and housing by pet stores, who really don't have the animals best interests at heart.