Hi - sounds exactly like what I had. Give it a day or two to calm down and ice it if you think it's helping, then get into eccentric heels drops (make sure you are warmed up before starting them - not straight out of bed or after icing) and start running short distances (2 - 3 miles) on the flat very slowly.
Keep up with the heel drops and gradually increase the distance and pace based upon how you are in the morning.
Oh, did I mention the heel drops? Do them. Lots of them.
I suffer from this occasionally and it freaked me out at first.
I had an ECG and a 12 point exercise stress test and all was normal. The guidance from the cardiologist was "it happens to some people, some don't even notice it. There's nothing underlying that's wrong that we can see. Keep running, try not to get stressed and your heart most likely won't be the first thing to go on you".
If you are not getting any pain or dizziness etc. then I'd relax and accept it for what it is.
A lot of the running I do is at a low level of intensity - by which I mean about 20 bpm below my lactate threshold. Obviously my average HR over a run can vary a bit (usually around 3 bpm either side of my target HR) and that will affect my speed. That can make it difficult to get a sense of where you are with regards your overall fitness without doing a tempo run or something (which is tricky where I run due to the number of tourists etc.).
I was pondering whether there was some way to correlate your HR and pace to come up with some sort of measure that could be used whether you were doing a tempo run or long slow run or whatever.
I've got some data going back 3 years or so and have worked out that average HR x pace (minutes per mile) gives a number that can be used to compare runs.
Here's a couple of examples....
28th Feb - did 5.8 miles, at 7.48 a mile, average HR = 127 = 68.79
4th Mar - did 7.3m, at 7.52 a mile, average HR = 123 = 67.19
I'm not that fit at the moment...so when I look back to when I was in better shape...
5.98 miles, at 6.37 a mile, average HR 140 = 64.33
So I know that for me, as the multiple gets down towards 60 I'm getting fitter....and it also means that I can predict to a certain extent what HR I will need for a certain pace or vice versa.....
Probably hopelessly flawed as a theory but worth a try...