A bruise appeared on my lower leg (shin) for no reason. what is the cause? should i see a doctor?

No: Bruising has occur with little or no trauma. Unless you have other issues with bleeding, I suggest you have a nice day and stop worrying.

YES: unexplained bruising may be related to a blood disorder- get examined and tested.

You decide: The shin, elbows & bony prominences of the body are all more prone to natural bruising with what may be minor/unremembered trauma. A bruise in the bodies soft parts, ie. belly, calf, backside, etc. would be unexpected & may signal a bleeding or platelet problem.