What Causes Swollen Hand With Dark Bruises On It?

i went to bed with a sore right hand so I rubbed it thinking it was tendons or pulled muscle. its in between my knuckles of my pointer and middle finger and runs down towards my wrist. this morning, it's swollen, extremely sore, and a faint dark bruise-like colour is on it. i have no memory of hitting it, hurting it or anything!