I'll start by saying I've seen a doctor and been through all sorts of heart tests with all results coming back as normal.
I'm getting quite an acute pain around the left side of my chest when I'm pushing my pace on 5ks. I'm a fit 35 year old man, been running for years and normal training runs of greater distance are fine, it's just when I'm lifting my pace or more accurately, trying to maintain my pace over the course of a 5k Parkrun that I feel a tightness and pain around the left side of my chest around the 2 mile mark which is impossible to ignore and causes me to have to ease off to a stop or to what feels like a crawl where the tightness/pain lifts after 30 seconds or so and I'm able to get back to my previous speed again.
I've noticed recently that it feels like I've aggravated something during these Parkruns because then setting off on a long slow 10 miler the next day has the pain returning very soon into my run, even though I'm obviously going significantly slower than what brought the pain on in the first instance.
I saw a physio and she gave me side stretches and some exercises to build up or muscle around my back but this has yielded mixed results and doesn't feel like a solution.
Many thanks for your help on this,