I can think of a number of things that could be going on. First and foremost is how do you feel? I've had lung function changes (ups and downs) lag how I feel. In other words, I'm feeling a lot better, but the lung function doesn't come up for another week or so. If you're feeling better, I would keep an eye on your lung function and hopefully it will come back up.
When I've been put on IVs for a drop in lung function, they usually start me at two weeks. However, they've always extended the run for me. Usually four weeks is enough, sometimes it takes longer. How did they confirm that the infection is gone? Usually, lab work takes a few days and my doctors won't stop the IVs unless I'm feeling better and/or my lung function has come back up.
Another thing to consider if whether they were treating the right infection. I have several bugs in my lungs and it's sometimes difficult to tell which one is causing me problems. They usually will try treating one of them with IVs and may switch to different drugs if I don't improve.
I had a drop in lung function a few years ago. After trying to treat the common bugs (e.g., pseudomonas), they finally did more extensive lab work and determined I had mycobacterium abscessus. I certainly hope this isn't the case for you, but maybe you've started to grow a new bug that requires different treatment.
I know it's very discouraging to have a drop in lung function. I would continue to talk with your doctor about options. I hope you're feeling better soon.
