Lungs and Tendonitis

I was a competitive gymnast for 15 years and wound up rotating 4 disks in my back as well as developing severe tendonitis in my left knee. 5 years later, I have now developed a condition in my left lung where it does not like to completely inflate nor deflate as it properly should (I was told it was because my tendonitis threw my hips out of alignment because I cannot completely straighten my left knee).
My question is, I am wanting to take a SCUBA class and would like to make sure that this is ok to do with my "lung condition"? Also, while my physical therapist explained it to me as best as he could, I do not completely understand the issue. All I know is that my tendonitis and inability to completely straighten my leg somehow effects my lung. Could you explain this a little bit further?

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