That does sound plausible.
Perhaps you can ask your mother about it or if you go with her during her visits you could ask to get a definite answer.
I wish all the best for you and your mother.
Keep us posted.
In researching CA 15-3 further, there are cancers other than breast (lung, ovarian, endometrium, bladder, GI) that can cause elevated levels. My best guess is that because my mother's NHL is deep in her left abdomen, the CA 15-3 and CEA are being monitored to keep a close watch on possible metastasis to the GI tract or stomach. Does that sound logical to you?
I am trying to figure out how the Oncologist is using the CA 15-3 and CEA markers; can't see how it ties into monitoring the NHL.