As you have mentioned , she is on chemotherapy, hence she will be having weakness as the cycles progress .
I will be in a better position to give you a response only if I know her blood reports and current planned chemotherapy. (I’m expecting chemo planned was AC 4th cycle )
If the symptoms are persistent , even after proper supportive care , it should be investigated for keeping line with investigation for progress of disease. But it’s very unlikely for the disease to progress while you are on chemo , but that may differ based on the type of cancer she had .
Next Steps
It’s unlikely for the disease to progress while on chemotherapy, so what you are seeing might be due to the side effects of treatment . But keep in mind that , some diseases based on its type could progress , so always discuss any unusual symptoms with your treating doctor .
Health Tips
I would to like to see her , post lumpectomy reports , ER , PR , HER2 status , planned chemotherapy and recent blood reports to give you a better picture online .