thank you for your query.
the pain in your chest is called angina
..and since it occured with running its called the effort angina..this is the stable variant which decreases once the activity is stopped..it is not associated with the blood donation
..i would suggest you to get a preliminary testing of ECG and Blood Pressure...the ECG will give us a better definition of the angina you are experiencing..kindly do not ignore and get the tests done..
in the meantime if there is a repeat you can take a nitroglycerine tablet sublingually(below your tongue) till you reach the ER..
wishing you good health