1st stage breast cancer was removed in march, had radiation therapy(30day), now taking tamoxifen, but coughing phlegm now, can it be a transfer to lungs?

Unlikely: Most likely, simple respiratory track infection- bronchitis, see your doctor for examination, may give antibiotics along with cough medicine, and possible chest x-ray., metastasis is unlikely diagnosis in stage i

Unusual: More likely is mild radiation pneumonitis - which can be detected on x ray. You might have a simple upper respiratory infection or bronchitis (especially if you smoke). It is unusual for an er+ tumor, stage i to metastasize that rapidly.

Not likely: It is more likely to have bronchitis or pneumonia in the lungs especially following treatment (radiation) for early breast cancer if you develop a cough with phlegm. Check with your doctor who might have you get a chest x-ray.