it could very well be her teeth.... all that excess drool can really screw up their little tummies and cause some really acidy, yucky (and painful) diapers!
i would treat it the same as a tummy bug though. only give her water (or very watered down juice) and skip the milk and all dairy until the loose stools (diarrhea?) stop. as long as she is having at least 3 wet diapers a day, dehydration isn't going to be an issue, so you don't have to worry about giving pedialyte (my kids HATE pedialyte!). go with the BRAT diet (bananas, rice, applesauce, and toast) for a few days.... those foods all seem to be good for settling upset tummies. stay away from milk and all other dairy until at least 48 hours after the last diarrhea diaper (it will NOT hurt her to go without milk for a week or two until her tummy gets back to normal!).
for her diaper rash, make sure she doesn't sit in a wet diaper long. rinse out the wipes completely, or use a soft wet washcloth, or wet paper towel to wipe her bottom.... some people just take their babies to the sink or tub and rinse them directly under the faucet. whatever works!
the quickest cure i know of is using a hairdryer on her bottom to dry up her bottom. a good method is to put her favorite tv show on and let her lay on her changing pad and watch tv while you hold the hairdryer on her bottom. just make sure it's on low heat and i always keep a hand right there so i can feel how hot it's getting. do this for about 30 minutes at least twice a day if she'll let you, and for at least 3-5 minutes with each diaper change just to make sure she's dry.
if she just won't let you hairdryer her bottom, at least use a soft dry towel to dry her bottom and let her air out for a good 5-10 minutes (or longer) before putting a new diaper on her.
the most important thing to remember is that a diaper rash will heal faster if it's allowed to dry out, so never put her diaper on her if her bottom isn't completely dry!
also, i would refrain from using any rash creams on her bottom (especially the open sores). do NOT use powder!!! if you think she just needs
it, use vaseline.... and again, make sure she's completely dry before applying, or you're just sealing in the moisture!
i just went through this with my two year old last week, so i really
understand what you are going through! good luck!