What Are The Multiple Bruises On The Legs Overnight?

This morning, I woke up with about six deep purple bruises on the outside of both of my legs. I assumed, because I had been carrying heavy drums the day before, that I had unknowingly been bumping them against my legs, but later in the day, the 6 turned into 13. I haven't done anything that would cause bruising and a blood test taken about a month ago showed nothing wrong. What's causing this?