My Baby yr old Aussie shepard hind gave out

[rimike]

My dogs hind legs just gave out first time ever and he could not get up for about 5 minutes although he tried He had a little saliva come out . He had just come in from going to the bathroom outside in the grass or muddy spot. About 20 minutes later he threw up a good amount of his food not projectile just regular What could this be? Does anyone know??

Thanks
Mike