Breathing heavily while cantering, should I be worried?

[Dumbo]

Just started canter work with my tb since he's come back into work. He's done at least 6 weeks of trotting, and now trots up to nearly 10 minutes at a time. No problems at all. This week we've had 3 short and steady canter's and his breathing is so heavy. The minute I bring him back to walk or trot his breathing is fine again.
The lady I rode out with says he must just be excited but I can tell it's not that - when he's excited I can feel his energy underneath me however when cantering he's so steady and listens to me.
Another suggestion is that he must be unfit still but he does a lot of trotting so I'm surprised a 30 second canter would cause such heavy breathing?
Is it something to worry about?

 

[AmyMay]

Has he ever had a problem with his wind before? I think I'd be concerned too that he was breathing so heavily considering he's been trotting for 6 weeks. However, it could be fitness - as 10 mins trotting is nothing. So maybe he's not as fit as you think he is??

 

[Goldenstar]

Are you sure he's not just high blowing which lots of TBs do it means nothing .

 

[Lucky Lady]

I am sure it is just a fitness thing as I too have TB race horses and this wouldn't worry me at all.

 

[Dumbo]

No other problems with his wind and nothing else that concerns me. Could well be a fitness thing then, it's just surprising that he only does this when cantering for such a short time but can trot quickly for a while with no breathing difficulties?