The best answer is to seek professional help. What we can only do is make suggestions and theorize. So lets take the symptoms one by one.
1. You remember the exact time the pain started so it is of acute onset
2. You breathe around 12 times a minute normally and you sense the pain sometimes(my guess is when you breathe deeply right?)
3. You write heart pain but you actually nean chest pain
at the left hemithorax.
Now for some questions:
1. With the scale from 1 to 10 where would you classify your pain?
2. how long does it last?
3. does it radiate anywhere in you body?(jaw upper arms, lower back)
4. Do you have the pain when you exercise or when you are resting?
5. Does it wake you up at night?
6. Do you have symptoms of the respiratory system? coughing, weezing?
You do realise that an accurate answer is not easy from the internet. But if your pains radiates, is strong, it lasts for a few minutes and doesn't go away with painkillers you should go to an ER department ASAP. It could be from your heart, it could be from your lungs, it could be from the thoracic muscles. Better test and be safe than sorry