My choc lab Charlie was able to hold it for 7 hours at 5 months old, with no problems. Since your dog has showed no problems before now, I seem to think there is another reason. Did you change foods, or start giving a new treat, or anything different? Or even a new bedding or removed bedding.
Rawhides??
As a matter of fact Charlie was not using his crate anymore at about 6 months. And still never had a problem. Now my yellow lab Shiloh was alot harder to train, and used the crate until almost a year.
But since your dog showed no problems before this, I would tend to look at other things, although you may really never discover why.....
I cleaned my crates by soaking them in the bathtub in bleach water, then rinsing well. Maybe there is still a smell in there left behind???
Anyway best of luck, you already got some good advice.
Julie.