I have an 8 month baby girl who has diaper rash. It is bright red with a few little blisters. What do I do?

Go see doctor. Sounds like an infection. It could be fungal or bacterial. Have your family doc or pediatrician check it out and prescribe the proper treatment.
Yeast/bacterial. Actually, there are several potential causes for what you describe, including a yeast infection, a bacterial infection (like staph, strep, or others), or a herpes infection. It could even be an irritant or allergic reaction to something that's been applied to the area. I think it'd be best to seek medical attention.