I'm so sorry about you losing Poinsettia(what a pretty name!) so suddenly. There are any number of reasons this could have happened and it is not uncommon that she continued to act "normal" until she died. Rats will often continue to eat and play despite illness. Because she was young(you don't say how old), she could have had a respiratory infection but symptoms may have been subtle and gone unnoticed by you. These infections can progress extremely quickly and cause death suddenly. Other possibilites(although more common with older rats) are kidney failure, heart disease, internal tumors, or cancer. Unfortunately, and sadly, you will never know. But please don't feel bad because it appears to did everything you could to provide the best possible environment and diet for your rats and there is likely nothing you could have done. I experienced nearly the same thing with one of my female rats when she was about 20 months old(probably a little older than yours). She was a fiesty one and was playing and eating as always. The next day we found her in a cage corner lethargic and laying on her side. I rushed her to my trusty exotic pet vet who told me she had lost a lot of weight since her last visit about 6 months prior, which I hadn't noticed because of her fluffy coat. The vet did not have an accurate diagnosis but felt a mass in her abdomen, possibly cancer. She died within an hour of my bringing her back home! I was devastasted also that this could happen so suddenly. But like you, I had always provided the best of care for my ratties, and realized there is nothing I could have done to prevent this.
I would not worry that it was a disease that was transmitted to her cagemates but check closely for breathing difficulties over the next few days. If you do have a good vet that is experienced in treating exotic pets, and you can afford a checkup, then by all means it cannot hurt to take them in. But again, commend yourself on giving them the best you possibly could and remember your baby with fondness.