Bruise the size of a golf ball on the outer side of my thigh, but I have no idea how I got it?

I sometimes get bruises on my legs and arms because I am a clumsy person naturally, but I woke up 2 days ago with a huge bruise on my leg, I actually only noticed it because it was so swollen, and still is a little actually. Its about the size of a golf ball and I have no Idea where it came from. I imagine to have swelling and a black and red bruise you would have really had to hit it good and I didn't, it wasn't there the night before and I didn't hit anything. should I be worried? I've heard that if you get big bruises and you don't know where they came from you could have a medical condition of some sort, or could I just be beating myself up at night? I thought maybe I was sleep walking but my parents said they didn't see me and I checked the height of everything in my room and none are the right height.