pressure? varicose veins kind of thing. one time i was feeling a tingly sensation and stinging painon one of my finger i could not see any bruising, but apparently the blood vessels kinda ruptured somehow, but i dont remember hitting anything. apparently they can just burst from pressure and cause a bruise without hitting something, or lightly hititng something.

Also i have this wierd bruise that appeared around the time my atopic dermatitis flares to, apparently theres inflammation related bruising.(it only flared one other time in '16).

of course there are systemic causes, but those are usually more, requiring a DR to diagnosis.