Given that you are exercising regularly, I think the most likely cause of your symptom of pressure on the left side of your chest is a strain of the muscles between the ribs on that side. This is a very common problem, which typically occurs in those who exercise, especially those who do vigorous exercises like push ups or sit ups.
I would suggest that you go to see your primary care doctor
about the issue. They can examine the affected area and confirm whether or not this is just a muscle strain
or something more serious. If it is just a muscle strain, then the usual treatment is straightforward - resting from the vigorous exercise, and taking an anti inflammatory medication like ibuprofen (as directed by your doctor) to relieve the symptoms.
In the meantime, if you have any more concerning symptoms that might suggest that this is a problem related to your heart or lungs (which I think is unlikely), you should seek immediate emergency medical attention. These symptoms would include chest pain, shortness of breath, fever
, cough with blood, fainting, or racing heart rate.
Make an appointment to see your primary care doctor at your earliest convenience!