We can feel tingling, changes in body temperature, tetany (pins and needles/numbness), subtle changes in the amount of energy in the body and not-so-subtle energy changes. In terms of my own early breathwork sessions, I had a lot of pain in my abdomen. Very intense spear like pain. This continued on for a least six or seven breathwork sessions where it gradually lessened then disappeared. From my reading it was related to some trans-personal story (past life or ancestral form of being wounded - literally!). This pain no longer occurs during my breathwork sessions.
Sometimes I have had sessions where I have felt totally leaden. Like I can't move my arm off the floor. Like some heavy tranquilizers have entered my system and I'll never escape them. But it does pass. This could be read as revisiting and integrating the drugs that crossed the placental barrier during the birthing process. A drugged mother generally also equals less quality bonding with the mother, hence less ability to bond with adults later in life.
Sometimes we just need to move our bodies. Thumping the floor, shaking the legs, moving the arms upwards, untwisting the neck. All these somatic responses are perfectly valid in a breathwork session.
So in summary don't be inhibited what your body wants to do in a session.