increase in Liters needed rising in just months?

Hi, my name is Lorraine. I have COPD and I quit smoking when I was 48 but at age 60 had my first exacerbation and was put on 2L of oxygen. At times I could run to the gas station without it but then I needed the 2 L whenever I exerted myself. I was fine without it when just sitting. I stayed on 2L and slept with 2L attached to my CPAP machine because my oxygen level drops to 74% when I sleep. I stayed on that for 3 1/2 years. Last June I got up to walking a mile on 2L then in July I needed 3L, then I had to walk really slow on 3L. In Aug, I ended up iin the hospital because I needed 4L so I thought I was having an exacerbation. I had pneumonia. They sent me home on 4L and said it would take awhile to get better but I ended up needing 5L just to stay at around 90% in Jan. My DR called in prednisone and antibiotics and a week later told me to go to the hospiital. They thought I had pneumonia again but another DR said it was just an exacerbation. By then I was up to 6L they said that was my new baseline. I got mad at my lung DR and told him how I felt and he walked out on my4 case while I was in the hospital. So I called to get my records and My breathing test last Sept. said my lungs were at 60% and mild emphysema. Does anyone know how I could get so bad so fast?? The DR's at the hospital don't know. Even when I had the breathing test. the results don;t equal a person being on 4L which is what I was on at the time. My Dr should have picked up on that... Any ideas? I have tried researching and can;t find any articles about that happening.