Vonners, as another migraineur, the idea of these shivers being a precursor to migraine rings very true. I have evenings when I feel bone weary for no reason, and I am so tired and exhausted I can barely make it up the stairs to bed. Like mrsmopp I then sleep heavily for longer than usual and wake feeling drugged. Sure enough by lunch time the migraine is there.
Mrsmopp you do not mention a headache but if you feels a bit spaced out during the following day it could be a headacheless migraine. They do exist and I have had them.