Jo You are right to be concerned and your mom is very lucky to have you following up on her care so diligently! Although I have no idea why her BP would increase after activities, my gut feeling tells me you should consult with a cardiologist or an internist to run some tests like an ecocardiogram, sonogram of the carotid arteries and whatever else he/she recommends. Thats what I would do.
In the meantime, maybe she should take it easy. Yogic breathing (Prana Yama) exercises do help.
Oh, how is her hemoglobin? I get heart palps when doing activities if its low and I feel like my heart is struggling. Below is link explaining the link between low HG and BP :
http://www.ncbi.nlm.nih.gov/pubmed/22949533