HI, I am out of town on vacation/business and the other day while lying by the pool I noticed a large lump had appeared on the top of my hand (the size of a half of a golf ball) I iced it and the swelling went down fairly quickly. Then it immediately began to show as a bruise which has spread across the entire top of my hand. I think I would have realized a bump to my hand that would have caused such a bump, but was just sitting on a lounge chair.
Looking at your history it seems to be a case of ganglion back of hand is a most common site for it to develop would like further details like onset of swelling any history of trauma any change in its size and shapes just collection of fluids in tendon sheath that can be Chat Doctor.