Call your pediatrician fast!!!! If this just happened, within 24 hrs, she maybe able to take a prescription that will limit the number of pox she gets. From the sounds of things, she's going to be covered from head to toe, and you don't want them to internalize, like up her nose, down her throught, or in her private parts as these are terribly uncomfortable. In the meantime, give her oatmeal baths, Aveeno or soda baths. Benadryl is usually for a histamine reaction and that's not what this is, so it may or may not help, you can try. Aloe might bring some relief.
As to where she got them, daycare, grocery store, etc. Your older daughter will probably come down with them as a result of her close contact with her little sister. Again, contact your pediatrician so when she does begin to break out you can give her the prescription to limit the number of pox. The ones on the face can scar and my daughter had them so bad, I swear she had pox on top of pox. All 3 of my kids and Dad ended up with them within the same 2 wk time period. I got Dad the prescription, because husbands can be bigger kids than kids. To complicate matters at that time, my Dad had lung cancer and this limited his ability to interact with the kids, so there were lots of phone calls and pictures being drawn between Poppa and the rugrats. Lucky me I had them when I was 5. However, I did get them again about 2 yrs after this round following a trip to Cincinnati. Aveeno, soda baths.... don't let her scratch them. Call the doctor. Best wishes for a speedy recovery!!