It could be you are "over-squeezed" by your wetsuit or buoyancy jacket as the above replies suggest. If so, bleed out a little air from your BC to see if that's it. If not, then could be wetsuit. Are you still floating nice and high out of the water? Let out a little more air and see what that feels like. If your wetsuit has a front zip, unzip it some and try that for feel.
But if it's not squeeze, could it be over-exertion, from trying to stay up "high" enough out of the water? That could happen if you are over-weighted (or under-inflated BC) and you're finning to stay "up", just as swimmers do when they tread water. Uses lots of cals and your breathing rate rises. Grabbing the side of the pool would fix that.
I'm guessing here. But I think it's one, or the other, assuming you don't have any breathing issues on shore with other activities.
Best wishes. You'll get it right, and asking questions is good.