Dukkha comes in many ways and forms. But the worse kind of dukkha are the uncontrollable cyclic type.
Non-stop breathing feels like a dukkha.
Non-stop heart beating feels like a dukkha.
In normal times, we hardly notice clearly that we're constantly operating like a machine for 24/7 since day 1.
In meditation, we become aware of these 2 activites. It gets to a point where even these 2 activites feel like a hassle, a constant burden which doesn't stop, even if we wanted to. You'd wish that you don't have to be constantly breathing. It feels tiresome. It's a wonder why they don't seem tiresome in normal circumstances.
You'd wish you could get some rest.