Ah, not going to explode. ;) At least for a loooooooooong time.
Anyway, it's probably the weather difference. Sometimes when it's hot right after a cold day or something, Lexi acts a bit unlike herself. ;)
But if the horse isn't acting normal otherwise, I'd get a vet out.
Can he be in pain? I would check with a vet to look for hoof tenderness, back pain, small grade lameness (I was surprised to see how a good vet can detect lamness in horses that I was sure they were OK. And I thought I can detect lameness...). Did you check his body temprature? Does he eat as usual?
All this points must be answered before blaming the weather..