Yorkie seems to be getting worse

We put a Hartz flea collar on our 10 year old Yorkie a couple of weeks ago and after about a week he started acting differently.  A couple days after we noticed that he started yelping because of stiff muscles and pain.  We thought he had arthritis.  He started walking around like a racoon with his back hunched up and his nose to the floor.  When he kept getting worse we decided to take the flea collar off in case he was having a reaction.  Within a few hours of removing it, he seemed to be getting better – except that he was scratching himself alot.  We thought that the fleas were bothering him so we put some Hartz UltraGuard Flea and Tick Drops on him.  Within a few hours, he started having trouble standing, he was yelping and his head started jerking (like a tick).  We got online and read about the side effects that seem to be killing pets.  We were shocked since we have used flea collars before and never seen this type of reaction.  We washed the stuff off him….. and shampooed him three times.  He laid around for several hours after that and is still having trouble walking this evening.  We are going to bath him again in the morning just to make sure that all the medicine is off his skin.  He has worn flea collars for the last few years and hasn’t had a problem – until now !!!  Our 11 year old cat is wearing one and has not had a problem with it…. but we are going to remove it before she does.