Unfortunately the problem with self-diagnoses and googling symptoms, is that you can misunderstand medical terminology and accidentally start to assume what you've been experiencing is exactly the same and inadvertently believe weird normal must be abnormal.
"Myoclonic seizures are characterized by brief, jerking spasms of a muscle or muscle group. They often occur with atonic seizures, which cause sudden muscle limpness."
You said..."I also have self-diagnosed myoclonic seizures at night when falling asleep. " but its a perfectly normal situation to experience startling jerks as you are falling asleep, which are not the same as seizures!
"Hypnic jerks or sleep starts are benign myoclonic jerks that everyone experiences sometimes in a lifetime. Although they resemble the jerks of myoclonic seizures, they occur on falling asleep and are just benign nonepileptic phenomena."
- NIHwww.ncbi.nlm.nih.gov › pmc › articles › PMC4481805
Shivering is an involuntary reaction when exposed to cold temperature, shivering is basically from small rythmic muscle contractions with or without the occational larger jerkier contraction too. Small rythmic muscle contractions are more like tremors until you warm up, whilst the larger muscle contractions will look and feel more like a startle jerk and these larger muscle contractions will replace the smaller rythmic muscle contractions the colder it gets or the longer you are exposed to the cold ie hypothermia.
Hope that helps.......JJ