What is the cause of his muscle spasm?

hi.i have a 10 year old male pomeranian.for the past few days,he has been down with muscle spasm.im not too sure if hes seizing.i took him to the vet recently for a check up and an xray of his abdomen was taken to observe if there was anything wrong in his abdoment.the vet found that theres a little bit elargement of his spleen.so she prescribed him with amoxicin and anti-spasmodic agent.today is his 1st day of treatment.after having his medicines,he is still 'seizing'.he doesnt jerk violently but it is something more like vibration.im quite worried.does anyone know what is exactly his problem?tq