Pay very close attention. I don't want to alarm you, it could very well be what they say above however................
When I first got my 2 clowns. One of them was very active, and would frequently twitch like that. The next day he seemed to be breathing heavy. The day after that he was dead. 3 months ago.
I just had a Tang die for no reason I could find,*( had him for 3 weeks ) then my Remaining clown started the twitching thing, as I looked closer I noticed the little white spots all over him. Not sure it if was Brook or Ich but he recieved a fresh water dip and now lives in a hospital tank until I am sure the main tank has run its course.
Again it may be nothing, but......... just keep an eye on em.