This can be a lot of things...you may be overtraining, not getting enough rest, sick, not having proper fuel, reacting to temperatures...there's so much this can be attributed to!
I recommend you go for a physical if possible just to rule out that it isn't anything major and in the meantime keep a running log and record things like how much sleep you got the night before, what the weather or temperature conditions are, your stress level, etc.
I know that when my puppy started really dragging on my runs, my heartrate would immediately skyrocket because it stressed me out dragging her along. Once I stopped taking her with me, my HR has been steady as she goes!
And of course - take care of yourself! Never push past the point of comfort!!!!
| Pounds lost: 53.6