1. I think it looked to be mostly from the back end.
2. No. He starts to walk and then puts his tail down and sits really fast. It looks like something bit in the butt. It is a very odd reaction. He doesn't seem to be in pain when it happens, but does seem bothered by whatever is going on.
4. None that I am aware of. He has had these weird tremors before but it has always been when he is asleep. His entire back end starts flopping up and down. He always stops when I pick him up. I have always assumed he is dreaming. He has never had any other major health issues. He is active and is eating etc.
There doesn't appear to be any swelling. He does have a mole just below that area (half inch below). It has been there for a year or two.