Iris,
Maybe consider taking a slightly longer break in between intervals until your body gets used to these types of workouts. Remember, in the Imaxes you get recovery and then a brief steady state routine before the next blast. As you know, the Hiits are all blasts with short breaks in between. Even if it doesn't feel like you're overly exerting yourself, consider that you are "shocking" your body with a totally different cardio experience.
Think about it, if you were experiencing CHEST pains and pressure doing the Hiits or any other intense workout, you'd probably stop out of concern for your heart. I'm not doubting your physical conditioning at all. I'm just sayin'... if it were me, I'd see any sort of headache or head pressure as a red light. It may be nothing, but better safe than sorry, right?
Another thing is to make sure that you are breathing properly. Suck that air in, as Cathe says, "it's free". If you're still really out of breath when the next interval starts, take a little more time, have some good old H20 and then resume the workout. I'm not advertising anything but I know for me, during a particularly intense workout, I drink electrolyte enhanced water.
For whatever the reason, for now, your heart may take longer to recover from the blasts than Cathe and the gang. I do my best to keep up with them but when I need more time, I take it. I imagine that they did not start out doing these Hiit workouts as flawlessly as did during the filming.
Definitely consult with your physician and in the mean time...
1) Listen to your body.
2) Hydrate, Hydrate, Hydrate
3) Keep breathing during the exertion phase and don't start the next interval if you're still really out of breath.
4) Consider slightly longer breaks and/or not going at it 110% (as we hard core Cathletes tend to do naturally).
HTH.
Fitness Friend,
Teals