Mysterious bruises...causes for this?

I recently obtained a bruise on my right bicep from when a 7 month old puppy playfully bit me on the arm. I knew where that bruises came from, obviously. The past week or so, I've had two bruises on my right forearm that I didn't know where they came from. I recently got one on my right forearm again that I don't know where it came from..the other two are disappearing, but they are still a little yellow. I now have a bruise on my right thumb and my right middle finger that I'm not sure where they came from. What could be the reason for this? I am a 21 year old woman.
 
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