It doesn't sound to me like your organic milk is ultra-pasteurized. Was the "box" refrigerated when you bought it? UHT milk would have been NON refrigerated.
I'd say try milk from another grocery store though. I'm suspicious about how they handle their inventory. I can't imagine how it would have anything to do with the organic part of it, unless their "non-organic" version has really weird stuff added. Yuck.