Tire Bulge problems of the 1999 Chevrolet Malibu

For no reason whatsoever a bulge developed in this tire. It grew on its own overnight to twice the original size when I discovered it. I took it immediately to the dealer and it was replaced with a similar tire. This replacement tire has 1991 stamped on it, the original for do not. I thought this was just a fluke, but with all the news about firestone, I must tell you I am very uneasy about driving with these tires. What if another bulge comes up and blows out, and causes an accident? I hope you will look into this because there are a lot of Malibus in raleigh with these same tires, (I checked). Thank you for your time, barbara dipert. (dot number: 93t mts tiresize: p215/60r15).