If something did fall on him I would bring him to your vet and possibly get some neck xrays to make sure nothing broke and caused the bruise that bad.
I could just be a bruise from something falling on him. But for it to flare up that quickly, specially since you checked his bod out a few days ago. Means something serious is happening and I wouldnt take it lightly. As I know you arent.
Call your vet tomorrow, and try and get him in as soon as you can. Mention the possibility of radiographs just to ease your mind. You can never be to careful with these small guys. Im sure their necks are fragile.
Keep us posted.
Does the skin look irritated and inflammed too or just like a bruise. Bc he could have gotten bit by something like a bug or spider, and this could cause an inflammation/ allergic reaction around the bite. Look for small punctures around it.
Just a thought. Im trying to think of anything I can.
And your right, it doesnt look anything like ringworm.