Your story sounds just like mine a month ago. My son slept 11-12 hours since he was 3 months old, then it just stopped. Sure, in the middle of the night, he'd get up and nurse, and I actually thought maybe he was hungry. Then it got worse, it went from waking up at 3AM to waking up at 12AM and 3AM. My husband and I tried crying it out, going in there and soothing him, just walking around the house with him. We began to only average about three hours of sleep.
Finally, my husband and I tried something new - a very, very strict routine. My DS does his last nursing around 6:30, bath at 7:00. Next, I read him the same story in the rocking chair each night. Then, I sing the same lullaby each night. Finally, I put him in bed and say the same words each night. The secret is to be extremely consistent and do the same thing each night.
Since the first day I tried this, he's back to sleeping 11-12 hours at night. He'll wake up and cry occasionally, but not for longer than 2 minutes.
I hope this might work for you!