18 Years old: Heart attack symptoms

Hi
I’m 18 years old and have been working out very hard for 3 weeks. Lifting weights 5 days a week and running 6 days a week (for nearly 40 mins a day). I’m decently fit and in pretty good health but this new work out was pushing my body beyond its limits. 2 days ago when I was running I felt some slight pain in my left chest which was radiating out into my upper left back and my left arm. I quickly stopped exercising and about 30 mins later the pain went away. The next day I went running again with slightly more intensity and had the same pain again (this time with pressure and tightness in the chest). I stopped immediately and just started to rest, yet the pain persisted for the entire day. The next day, the pain in my heart was almost gone but I still had some slight pain and pressure/tightness in my upper left chest and left arm (which disappears and reappears every once in a while), and the pain in my heart becomes slightly noticeable with any activity which makes my heart work a little (even moderate-fast walking). I know heart attacks are very uncommon for an 18 year old, but these symptoms are quite worrying. Also I have no history of heart disease in my family. The good thing I notice is that the conditions are slowly improving (the pain is pretty much completely gone as I write this, but still comes back a little occasionally.) Oh yea, I also had a little nausea earlier.
How serious does this situation sound? What should I be doing about it? I am quite nervous about this and will be seeing the doctor in a few weeks (currently out of the country.)

Thank You for your time and consideration