I'm very sorry about your dog.
Gastric Dilatation and Volvulus, also known as "Bloat", is a potentially fatal condition. Even with dogs who receive treatment, as many as 25% to 40% die.
Gastric dilation starts with a distended, swollen stomach. The stomach is usually full of air, and is so full that the dog can't empty it's contents by belching, vomiting, or digestion.
The condition is called gastric torsion when the stomach suddenly twists up to 180 degrees. At this point the twisting causes damage to tissues and organs.
Gastric volvulus is when the stomach twists from 180 to 360 degrees. At this point the stomach is closed off at the esophagus and the small intestine. Ischemia, the lack of blood flow, quickly causes necrosis (death) of tissue. Soon bacteria from the dead tissue gets into the blood stream and causes massive infection or septic shock. The stomach is still distended, which puts pressure on the lungs, causing difficulty breathing. The distension also puts pressure on the blood vessels, which slows the blood return to the heart, resulting in lowered blood pressure. As there is less blood volume for the heart to pump, heart function is compromised and arrhythmias may occur.
The spleen can begin to bleed because the twisting of the stomach causes it to separate from the stomach. This may cause clotting abnormalities to develop, and the dog can bleed to death very quickly.
When Gastric Dilatation and Volvulus occurs, it can take less than an hour or up to 36 hours for it to result in death.
Since in your dogs case, her spleen was folded upon itself, it must have separated from the stomach, causing her to "bleed out", which can quickly result in death. If I had to guess, and it really would be a guess, I'd say she would have died within an hour from her spleen separating from her stomach.
Vets do not really understand what causes GDV in the first place. All dogs swallow air, but for some reason dogs that develop GDV cannot release this swallowed gas. GDV is more prevalent in some breeds than others, although all dogs can get it. (English Bulldogs are not one of the breeds it is commonly found in.)
I had never heard of GDV following a caesarean, but after doing some research I found an article about the risk of GDV developing after the use of a specific anesthesia during surgery. These were the results of the study:
A recent study has described the development of gastric dilation due to aerophagia (excessive swallowing of air) during the recovery of a dog anaesthetised with tiletamine/zolazepam.
I would ask your vet what anesthesia he used. GDV is just not common after c-sections or surgery. I think your vet should also explain to you how he thinks the GDV occured, and why it wasn't noticed post-op. There may be legitimate explanations for these questions, I just think you have the right to understand what happened.
Again, I'm very sorry about your dog, and I hope this information helps. Don't hesitate to ask if you have anymore questions.