I am always getting bruises on my thighs, it is from bumping into tables, desks, beds, etc. but i never know where the bruise came from, maybe you are having the same issue?
On a somewhat unrelated note:
I used to wear 4" stilettos everyday, I mean I worked in a bookstore, so they were totally practical, anyway, one day I got this awful foot cramp and could hardly walk. Later on it developed into this bruise that was on the top and bottom of my foot, basically it went right through the middle. It was bizarre. I started wearing flats after that.
I used to get a lot of random bruises. Its caused by several things… thin blood (or prescribed blood thinners), hemophilia, or even taking NSAIDS, aspirin, etc. Any bruising of unknown origin should be suspicious… Outside of what I listed above only its mostly just liver disease, cirrhosis, etc and I doubt you have either of those…