Ask your mom if she has what feels like she can't quite get enough air into her lungs? This might be Babesia - a common co-infection.
If on the other hand, it's more like her chest is sore - in between the ribs or the sternum - this might be Bartonella, yet another common co-infection. But, since the oxygen helps it, I would assume it's the air hunger (can't quite get enough air) from Babesia.
It's actually quite common for other infection symptoms to become more obvious/pronounced as the bacterial loads for one of the infections she has lowers. We refer to this as the onion layer effect - as you peel away one infection, another one "shows" up.
Detoxing more may be of help still though.
And, congratulations on the few days of improvements!!!
That's really wonderful!!!
As the others have said, to have a few good days then to return to the way things were is hard to deal with, but it's quite common for these infections.