I am moderately active, normal blood pressure, fairly average weight and build. I've noticed that when I'm running in cold weather, about 5-10 minutes into my run I start to get what feel like heart palpitations, a somewhat uncomfortable but not painful fluttery feeling in my chest. I don't get dizzy or anything, and am able to run normally, but I usually stop for a few minutes so I can make sure everything is ok.
I have had similar experiences when I used to work in a chilly building (65-68F) all day then walk out in the summer air (>95F). For the first few minutes I'd get the feeling my heart wasn't beating fast enough, and it had the same fluttery feeling, but once I take a few breaths and give it a few minutes, things seem to normalize.
Why would this be happening? Do I have bradycardia? Could this be dangerous?