What causes extreme fluctuations in blood pressure?

My aunt is in a rehabilitation facilty to recover after a nasty fall down the basement stairs. She has a history of hypertension and is taking medication for it. Lately though her blood pressure has been rising drastically and then falling incredibly low. It's not related to her position, time of the day, or activity. I know I read something recently about these kinds of symptoms, but can't for the life of me remember what caused it! She's 85, with thyroid issues, atherosclerosis, ibs, hypertension, ocd, and some blockages in the vessels that supply the heart muscle.