I Have No Pain, But Suddenly Noticed A Bruised Area

I have no pain, but suddenly noticed a bruised area and some inflammation on my inner left wrist. I'm right-handed, and didn't experience a known blow or injury. It's fading one day later, and the inflammation is less, but it's still visible. I'm 65...normal blood pressure. I doubt it would be relevant, but I did have an out-patient knee surgery over a month ago, removing 2 screws a year after a fractured patella.