Hello Kristina ,
You have written that you feel tired and heaviness in chest.
As you had Asthma that was relieved at around 7 years of age, there is a definite chance of asthma coming back at 14 years of age.
This can be assessed by pulmonary function test ( PFT ).
Other possibilities in your age are Anaemia
( low haemoglobin
in blood) and heart disease
These can be evaluated with Haemoglobin estimation and Echocardiography
Your family physician would guide you further on this issue.
Do talk to your parents on these issues