Zero difference. It's considered the booster. The exact same content & dose as the first. I know...it's got me scratching my head as well
Well, the point of vaccination is to develop the body's adaptive immune system to recognize and attack a specific antigen. Days after the first shot, your body has developed the antigen-specific T cells, B cells, and antibodies. So the second time the body encounters the antigen, those components of the body's adaptive immune system are locked and loaded and react very aggressively. Those T and B cells then pump out lots of cytokines which then leads to an inflammatory milieu and the type of symptoms you describe.
And it makes sense for the body to act this aggressively. Usually, the adaptive immune response would ensure that exposure to a previously fought virus would be very swift and prevent its growth in the body. So naturally, large amounts of the viral antigen would never occur if the first adaptive response was successful. The immune response is tuned to only fight small amounts of antigen, but when large amounts of them are injected into the body at once as part of a second vaccine shot, then the body falsely reads that as the infection being out of control and will overreact. Part of that may be autoimmunity.
So I would much rather take a vaccine where there is only onetime exposure, or where the shots are spaced apart for long periods so that the adaptive immune system has lost some of its potency (as with flu shots where the shots are spaced a year apart, and where the antigens are somewhat different year to year).