Chihuahuas

It started about3 days ago.My family and I got home from being over at my mother-in-law's house for 5-6hrs.I opened the door and Chico was there waiting.My wife sat down and Chico sat right next to her like he always does.He was acting strange.My wife drop the remote he started yelping.He is not walking right at all.I checked him out.I touched him all over to see if there were any tender spots but there were none.When he walks he almost drags his butt.I gave him a bath the other night and he kept falling over..most of the time the his trying to climb out of the sink.He hasn't been jumping up on the bed like he did..not at all.Someone told me this breed has bad backs.Does anyone think he could have had a disk go out?Does that happen with dogs?Any info on this would help me out alot...THANX