YANBU to freak and this needs to be followed up with a frank talk with your dp about the avoidable risks of leaving a very young child alone for even a few seconds.
Or even, come to think about it, taking one's eyes off them for a few seconds. Had just got dd out of the bath and was helping her brush her teeth, glanced down and ds (just under a year) had silently slipped backwards and was laying in the bath with face out of the water as it was shallow.
I still shiver when i think of what might have happened if he's had fallen face down.