Nose breathing causes side pain?

Ran 46 minutes (approx 3.2 miles)

For the first couple of miles it took no effort. The last half was harder. I've discovered, I think, that trying to breath exclusively through my nose makes my side hurt. My side did not hurt today although I thought I felt a twinge the last mile. The key is controlled breathing, so I read. I also tried to exhale on the left foot strike.

The temperature was 76 degrees and the humidity was 91%. Another factor in my run taking more effort.