Question about horse cramping??

Yesterday I took Sassy's blanket off and let her run in the arena for awhile. She ran more, and harder, than usual, in fact I finally had to yell at her to Whoa so I could catch up to her and put her back in her stall.(She was playing keep away)

This morning when I went to ride there was note from the girl who rides her some evenings, that Sassy was fine last night until she asked for the canter - she wouldn't canter and then broke into a sweat and started shallow respiration. THey gave her Banamine.

Today she was fine. I couldn't keep her trotting the bounces, she wanted to canter. No hitch;

so what do you think> I'm going to guess butt muscle cramp or something like that. DO you think I should not have let her run so much in the cold? SHe didn't work up a sweat and I blanketed her after letting her roll and then brushing her.

I wonder if I did something wrong.