Why am I feeling this tightness in my upper calf?

Have just started using a foot pod with my Garmin in an effort to monitor/increase my running cadence. I was able to hold an average of 92, max. 95, min 89 for 40mins and I noticed I had to shift my style to manage this. I am wondering if it made me heal strike, as now I have very tight upper calves, which I normally don't have at all. Any ideas if this would be related and how to stop it happening next run?

Thanks in advance for your suggestions/comments please.