Finding HIIT workouts really tough lately

So I'm no exercise amateur and have been in the habit for well over a year... I've recently completed two F.B programmes and am perfectly healthy though I am taking iron supplements to give some background.

I'm finding HIIT workouts really tough! I get dizzy and just can't seem to push myself. I've been skipping on the HIIT and doing yoga, pilates, running or any other long-type cardio. I just wondered if anyone had any advice? HIIT just feel way too tiring lately