The only movement in my body was/were my trembling legs and heaving lungs.>
Should it be was or were? Grammarly tells me is was but to my me were sounds more logical since legs and lungs are two items.
Your choices are
The only movement in my body was my trembling legs and (my) heaving lungs.
The only movements in my body were my trembling legs and (my) heaving lungs.
The second would sound more correct.