B.
It is quite likely you will never know why your membranes ruptured. I had a very similar situation with my first child. I had a completely normal, extremely healthy pregnancy with absolutely no complications. Shortly after my 28 week check up, I developed a high, slow leak in my bag of water. It was so slow that I didn't initially think anything was wrong because I felt fine. At 29 weeks I went in to have it checked, thinking it was nothing, and found out that I had ruptured and was having regular, painless contractions. I went immediately to the hospital and stayed there for another week until my son was born at 30 weeks.
I know exactly how you feel. It does feel like a nightmare to have everything change so suddenly and to go from a totally normal pregnancy to a very high risk one in a matter of minutes. It is quite a lot to adjust to. The fact is that it is out of your control and sometimes this just happens. You didn't do anything to cause it.
All you can do now is follow your doctor's advice and try to prepare yourself for a different first few months than you expected. As hard as bedrest in the hospital is, it is easier than having a baby in the NICU (I know, mine was there for 7 weeks). Allow yourself time to feel shocked and scared and sad. Those are normal feelings!
And do take comfort knowing that, though the first months will be tough, preemies do remarkably well. My son is now 3 1/2 and you would never know he was so premature. He is a healthy, happy, strong little boy.
Feel free to email me privately if you have more questions. I really needed to hear success stories and talk to other moms who had been through something similar when my son was born. You will get through this and it will be ok.
Hugs,
B.