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You probably just bumped into something without noticing, it’s probably nothing to worry about.
Though bruises appearing out of nowhere, by themselve, may indicate a serious illness (I’m not gonna mention it so you don’t get spooked), so it’s best to see a doctor and do some blood work, including D-Dimers.
It’s basically an indicator if your body is producing coagulants in higher ammounts than it should be.