ulcer pre-existing or a result of the RNYGB?

I had a roux-en-y gastric bypass on Dec. 20, 2010. One evening, three weeks after surgery, I felt light headed after standing up and thought my blood sugar had dropped. Shortly after standing, I passed out. I woke up very weak and unable to stand. My husband came home to find me laying on the kitchen floor in a pool of blood and called 911. My blood pressure had dropped severly. I spent several hours in the ER and was given a transfusion due to blood loss. I was scoped by a gastroenterologist who determined I had a stomach ulcer where the stomach meets the small intestine. The bleeding continued a few days and I was also given a colonoscopy (which was negative). Omeprazole was prescribed for twice a day. I quit smoking over 15 years ago and have been following my surgeons advice for eating post surgery. Was this ulcer pre-existing or a result of the RNYGB? What else can I do to avoid a repeat of this episode? I had already groomed byself for a lifestyle change, but so unexpected.