When i am tired, i feel like i'm not getting enough oxygen and my lungs won't expand fully. is this a normal function?

Don't worry: You're still young to have serious lung condition. Have a spirometry and your oxygen saturation checked in your dr's office to check for asthma or other obstructive lung disease. Quit smoking if you do. Take slow deep breathing every now and then. Exercise will improve your lunng function.

Yes: It's not unusual for me to see patients who are tired/ not getting much exercise who feel a need to sigh deeply, feeling they can't quite get that deep breath they need to satisfy them. The only two things I have found that help them are more rest and more exercise. There are some stretch receptors in the lungs which i believe are playing a role in this problem.