I get the same thing when I take an incredibly satisfactory pee. I'm not even kidding. The reaction is actually called "post-micturition convulsion syndrome" and it's thought to be because your body drops blood pressure to make you pee, and then you release chemicals and hormones to fight the drop in blood pressure. Also, something about the "fight-or-flight" senses and the area of the brain responsible for that.
Maybe your body has conditioned itself to associate weed with lowered blood pressure (which it does trigger) ? Maybe you're just subconsciously excited to get some cannabinoids. I know when I have a really bad sweet tooth from my Vicodin, whenever I fulfill the specific craving, it's just
so much better than if I were to have it normally. Sometimes it's a 30 minute drive to a specific Greek food place because of the cucumber sauce they use, sometimes it's the Snickers I have in my car. Once I can fulfill that craving, my body feels... crazy. All giddy, electrical. Like I'm full of static being released in a really weird, primitive way.
Maybe I'm just really high, I keep feeling the shivers