Thanks for your advice - I am a bit anxious - I also feel like a bit giddy at times which is probably another sign of this.
Regards
Heidi
The severe asthma attack you had was clearly, pretty scary, and from your description, has left you understandably anxious about your breathing. You might want to give some careful thought to the circumstances surrounding your asthma worsening, especially with regard to environmental exposures, including both inhalants and food, especially shellfish.
The dull back pain is probably musculoskeletal pain, related to labored breathing, during the attack and nothing to worry about.
It would be good to get a follow-up chest x-ray to confirm that the "slight infection" has resolved and your x-ray is now clear. Assuming that your asthma is now under good control and your chest x-ray is now clear, the need to "catch your breath" and your awareness of not being able to "breathe very deeply" are probably a reflection of anxiety.
It could be helpful for you to monitor your breathing with a peak flow meter. You should check with your doctor on this. The best way to "build up your lungs" is to engage in a regular aerobic exercise program.