My 11 Year Old Chihuahua Is Having A Spasm In

My 11 year old Chihuahua is having a spasm in either his back or back legs. He falls down and starts hollering loud and it looks like he is having a spasm in his back or his legs. It lasts for a couple of minutes and when he finally stops, at first he is a little weak in his back legs. After he stopped hollering. he is okay. This has happened two times.