I wear a HRM to monitor my fitness and recovery. My resting heart rate is typically 50-55bpm, increasing to 60ish the day after a hard training session.
Today it's been less than 40 all day and now 31. Which is off the scale, apparently the fittest athletes are around 40!
Does it matter, or is it only an issue if it drops to 0? 
Fwiw I've felt dreadful all day, not ill exactly but like all I want in the world is to lay down.