I agree a trip to the pedi is in order because it can be from anythin from foods to an infection.
If it does turn out to be a diaper infection - my daughter is super sensitive and here is the only thing that worked for us (hers would flare into a yeast right away)
eliminate baby wipes unless you have a big mess to clean up. instead wash over the area with tepid water. use a soft cloth (i cut up a cloth diaper into squares) and don't rub the area. pat dry and make sure the area is fully dry. we have been able to go back to sentitive baby wipes once the area healed.
alternate between a layer of 1% miconozle (sp?) and the next time only a layer of cortizone cream with each diaper change. wait for it to soak in and coat with a layer of A&D (target store brand is our fave).
Use a good diaper and you may have to try a few. we have gone through everything we can only have pampers or seventh generation.
Add probiotics to her diet. Florastor (available behind the pharmacy counter) is the best. if you are ever going to take antibiotics this is a must. you may not want to ive it long term as it is expensive. a good yoghurt with active cultures is a good on going alternative. (you can also get Kefir in whole foods....like youghurt...better live cultures).
Keep a food journal. for us my daughter was sensitive to citrus and fruit juice so i had to eliminate citrus for a bit and serve her juice 50% water.
Finally the vinegar in the bath might cause some other problems. again tepid water is the least painful during a flare up. Aveeno makes a good oatmeal bath. they come in packets and easy to keep on hand.
we were finally able to clear things up this way and not have it happen any more. Again we have a super sentive skin to deal with and your situation may be different but if it is a diaper rash these things should help heal her up quickly.Good luck!!!