hello and thank you for asking HCM, I understand your concern. Some people, especially women and older people, are more prone to bruising than are others. As you get older, your skin also becomes thinner and loses some of the protective fatty layer that helps cushion your blood vessels from injury. It could be that your mum hit her hand during sleep or that someone tight squeeze her arm. If this bruise doesn t vanish in a couple of days, or if you discover an another bruise, than visit your GP and make a blood test to see if everythink is ok. I hope this answer was helpful to you.