Grunting and grinding teeth

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I'm on my way to the trailhead and pulled over to check on the boys and one is grunting or moaning quietly and grinding his teeth a little. 

They have been strictly eating grass hay with some apples and pears for treats. But today before I loaded the boys I gave them some alfalfa to give them some good energy before the hike. Could he just have a belly ache with the wuick change of diet. 

I gave each a 1/3 to a 1/2 of a leaf of alfalfa 

Any ideas or suggestions?

Thanks