Ganglion cyst. You didn't mention pain, just the presence of a strange lump on the back of your wrist. This is a classic presentation for a ganglion cyst which is just fluid collection in a pocket of joint lining or synovium. These are benign & don't spread. They can be aspirated (fluid is very thick) or we can poke a bunch of holes in cyst and let it drain on its own. Sometimes we just drop a book to break it!