Poor you. My mum was like that and half the time didn't know where the bruises came from, nor remembered knocking herself. I seem to remember her having a blood test because she'd been concerned that folk may have thought that dad was walloping her. Nothing was wrong. I can also remember her saying " where did that come from ".
Yet I'm so different in that respect as I was told years ago when I began taking warfarin that I'd bruise easily after the slightest knock. Not a sausage, not that I want a show of bruises but there's rarely been a sign, even after a door closed on my arm----a bit of a red mark and that was it. I've got skin like a rhino I was in my 70's at the time.
It's usually put down to age but as I remember, mum wasn't old at the time. Sometimes it could be medication, or even something in the diet that causes the blood to thin. Alcohol.