Tami,
This is only a guess but I suspect the CXR was ordered to evaluate whether your mother has aspirated (taken into the lungs) anything while she vomited or while she dealt with the thick mucus. Aspiration pneumonia, if diagnosed, needs to be treated. Hope she is improving and she becomes more comfortable.
Janet

Caregiver, Husband diagnosed 7/6/05 SCC left tonsil, T2N2bM0, Concurrent chemo/radiation begun 8/2/05 (Cisplatin). Received 2 cycles cisplatin, 38 radiation treatments. Completed course 9/27/05.