Make sure you're using all natural, eco-friendly diapers and creams. I recommend The Honest Company. Their diapers are made without harfmul chemicals and their cleaning products, bath products, etc. are all organic and very gentle. Their healing balm works really well on diaper rash. Since The Honest Company is a mail-order company only, I recommend placing an order today, but also going to Babies R Us to get new diapers (Earth's Best are good) and new diaper cream (Method, Yes to Baby Carrots, and BabyGanics are good brands) for sure, and maybe new fragrance free laundry detergent (BabyGanics). Look for products that aren't tested on animals. Those will made with plant ingredients and no chemicals, which can be irritating. Beyond that, has she started eating solids yet? If so, she could be having an allergic reaction to something she ate. This could also be the case if she is on formula, or if she's nursing and you ate something that doesn't agree with her.