Our cat Henry awoke from a nap and when he started to walk he was very wobbly?

within a few seconds he started to slowly move in a circle to the left almost like he was chasing his tail in slow motion.He was very wobbly doing this and and his head was moving as though he was looking at something.I knew he was out of it as he didn't respond to us at all.This lasted for at least 10 to 12 minutes and as I frantically called the vet my husband picked him up and held him so he wouldn't hurt himself.He was also purring extremely loudly thru the whole episode.The vet asked me a few questions and I described to her everything Henry was doing.Its Sunday and the office was closed but the answering service got a hold of her and she called me within minutes.She said that it would take 20 minutes for her to get to the office and that Henry's episode would be over by then.She had me check his ears to see if they were red inside or black(indicating ear mights)They both looked fine so she said that was good.She said that it could be caused by a virus which they don't know much about and has no known antidote.You just ride it out.Or various neurological disorders.Keep an eye on him and don't be surprised if he has another today as that is normal.She didn't even want to see him tomorrow if he appeared fine,which he does.I just found the whole thing very frightening and felt helpless I couldn't help him.He never frothed at the mouth or had any sort of violent fit.Just what I described.I will definitely take him in the morning whether she feels its necessary or not,but has anyone ever had this with their cat or does anyone know why this happened.It took about 30 minutes for him to be back to normal,and now 5 hrs later you would never know anything happened.Sorry for being long-winded I just wanted to explain things fully.