I get those shivers constantly, not just in conjunction with tryptamines. For me it is a large, full body twitch accompanied by a flush of goosebumps, sometimes all over, sometimes confined locally to an area. I have had it happening for about 6 years now, although I do attribute it to changes in my energy body due to use of psychedelics in a broader sense. I feel them to be almost another sense being activated, meaning that I have become more sensitive to etheric energies/beings over the years and that these shivers are reactions to these types of energies, which are around us all the time but more prevalent when one has taken psychedelics/tryptamines.