I would have thought walking into something would cause one bruise, not three little ones, and you clearly don't remember the impact. That sounds pretty extensive for something you didn't even notice. Do you take aspirin or anything else that affects bleeding? Could they be insect or tick bites? You say you arrived with them, and you have a habit of sitting with your legs under you. Could you have squashed the blood vessels while travelling or while sitting around at the airport? If it expands any further, I would see a doctor. In fact, I would consider seeing one anyway, it sounds unexpected and undeserved and might be serious.