I'm not a medical professional, keep that in mind. I'm exercising on a treadmill, wired up with a three lead monitor in a hospital rehab setting following a cardiac event, and I've learned that my habit of exercising at full bore with little warm up time should change. If I don't warm up, my heart rate really spikes right after the gitgo. I'm taking a Beta blocker so my heart rate has a 'governor' on it, but it still goes up too fast without a warmup. If I warm up, the spike doesn't occur. That's just a thought, I'm an older geezer than you, though. I notice when lifting weights and wearing my Polar heart rate monitor in the gym, exercises like a sitting arm curl really spikes the heart rate, but sitting T-bar presses which work the larger muscles of the back don't bring up the heart rate near as quickly.
I didn't mention warmups, but I spend 8 minutes on the treadmill walking before stretching and then heading out the door. I just went back and reviewed my logs (I use a Garmin Forerunner 305 - HIGHLY RECOMMENDED) and can see all my runs on my laptop. This stopped last May, then started again in the middle of September. I wonder if it is cold weather related. The heart wants to keep the body warm... I live near Chicago and it is now in the 30-40's here at 4:00 am.